Paludan in his work (The Presidency of Abraham Lincoln) stated that the problem was the word used, blockade.
Nations do have a right to close their ports.
Jefferson did so when he tried to keep us neutral during the war between France and England.
http://www.wideopenwest.com/~jenkins/ironclads/proclams.htm
By the President of the United States of America.--A Proclamation.
Whereas an insurrection against the Government of the United States has broken out in the States of South Carolina, Georgia, Alabama, Florida, Mississippi, Louisiana, and Texas, and the laws of the United States for the collection of the revenue can not be effectually executed therein, conformably to that provision of the Constitution which requires duties to be uniform throughout the United States; and
Whereas a combination of persons engaged in such insurrection have threatened to grant pretended letters of marque to authorize the bearers thereof to commit assaults on the lives, vessels, and property of good citizens of the country lawfully engaged in commerce on the high seas and in the waters of the United States; and
Whereas an Executive proclamation has been already issued requiring the persons engaged in these disorderly proceedings to desist therefrom, calling out a militia force for the purpose of repressing the same, and convening Congress in extraordinary session to deliberate and determine thereon:
Now, therefore, I, Abraham Lincoln, President of the United States, with a view to the same purposes before mentioned, and to the protection of public peace and the lives and property of quiet and orderly citizens pursuing their lawful occupations until Congress shall have assembled and deliberated on the said unlawful proceedings, or until the same shall have ceased, have further deemed it advisable to set on foot a blockade of the ports within the States aforesaid, in pursuance of the laws of the United States and of the law of nations in such case provided. For this purpose a competent force will be posted so as to prevent entrance and exit of vessels from the ports aforesaid. If, therefore, with a view to violate such blockade a vessel shall approach or shall attempt to leave either of the said ports, she will be duly warned by the commander of one of the blockading vessels, who will endorse on her register the fact and date of such warning, and if the same vessel shall again attempt to enter or leave the blockaded port she will be captured and sent to the nearest convenient port for such proceedings against her and her cargo as prize as may be deemed advisable.
And I hereby proclaim and declare that if any person, under the pretended authority of the said States, or under any other pretense, shall molest a vessel of the United States, or the persons or cargo on board of her, such person will be held amenable to the laws of the United States for the prevention and punishment of piracy.
In witness whereof I have hereunto set my hand and caused the seal of the United States to be affixed.
Done at the city of Washington this nineteenth day of April, in the year of our Lord one thousand eight hundred and sixty-one, and of the Independence of the United States the eighty-fifth.
[L.S.]
Abraham Lincoln.
By the President:
William H. Seward,
Secretary of State.
Proclamation of the President of the United States regarding extension of blockade to the ports of Virginia and North Carolina, April 27, 1861.
Whereas, for the reasons assigned in my proclamation of the 19th instant, a blockade of the ports of the States of South Carolina, Georgia, Florida, Alabama, Louisiana, Mississippi, and Texas, was ordered to be established; and, whereas, since that date public property of the United States has been seized, the collection of the revenue obstructed, and duly commissioned officers of the United States, while engaged in executing the orders of their superiors, have been arrested and held in custody as prisoners, or have been impeded in the discharge of their official duties without due legal process by persons claiming to act under authority of the States of Virginia and North Carolina, an efficient blockade of the ports of those States will therefore also be established.
In witness whereof I have hereunto set my hand and caused the seal of the United States to be affixed.
Done at the city of Washington, this twenty-seventh day of April, in the year of our Lord one thousand eight hundred and sixty-one, and of the independence of the United States the eighty-fifth.
[L.S.]
Abraham Lincoln.
By the President:
William H. Seward,
Secretary of State.
Message to Congress in Special Session
July 4, 1861
Recurring to the action of the government, it may be stated that, at first, a call was made for seventy-five thousand militia; and rapidly following this, a proclamation was issued for closing the ports of the insurrectionary districts by proceedings in the nature of Blockade. So far all was believed to be strictly legal. At this point the insurrectionists announced their purpose to enter upon the practice of privateering. Other calls were made for volunteers, to serve three years, unless sooner discharged; and also for large additions to the regular Army and Navy. These measures, whether strictly legal or not, were ventured upon, under what appeared to be a popular demand, and a public necessity; trusting, then as now, that Congress would readily ratify them. It is believed that nothing has been done beyond the constitutional competency of Congress.
a proclamation was issued for closing the ports of the insurrectionary districts by proceedings in the nature of Blockade
He meant to say that he proclaimed a blockade. He proclaimed a blockade, not a closing of the ports. He did eventually get around to closing the ports -- on April 11, 1865, four days before he died.
Just before the war ended, Lincoln issued an order closing the ports, rather than blockading them.
From Gideon Welles, Lincoln and Johnson, First Paper, Galaxy Magazine, April 1872, p. 523
Mr. Seward, who had been uneasy since his return, [nc: Seward had been thrown from his carriage and injured] read to the Secretary of the Treasury and myself the draft of a proclamation he had prepared for the President to sign, closing the ports of the Southern States. This was a step which I had earnestly pressed at the beginning of the rebellion, as a domestic measure, and more legitimate than a blockade, which was international, and an admission that we were two nations.* * *
The President reached Washington on the evening of Sunday, the 9th of April. When I called on him the next morning he was in excellent spirits, the news of Lee’s surrender, which however was not unanticipated, having been received. While I was with him he signed the proclamation for closing the ports and expressed his gratification that Mr. Seward and myself concurred in the measure, alluding to our former differences.
OFFICIAL RECORDS: Series 3, vol 5, Part 1, page 107
(Union Letters, Orders, Reports)
VII. April 11, 1865.-Closing certain ports.
BY THE PRESIDENT OF THE UNITED STATES OF AMERICA:
A PROCLAMATION.
Whereas, by my proclamations of the nineteenth and twenty-seventh days of April, one thousand eight hundred and sixty-one the ports of the United States in the State of Virginia, North Carolina, South Carolina, Georgia, Florida, Alabama, Mississippi, Louisiana, and Texas were declared to be subject to blockade; but whereas, the said blockade has, in consequence of actual military occupation by this Government, since been conditionally set aside or relaxed in respect to the ports of Norfolk and Alexandria, in the State of Virginia; Beaufort, in the State of North Carolina; Port Royal, in the State of South Carolina; Pensacola and Fernandina, in the State of Florida, and New Orleans, in the State of Louisiana;
And whereas, by the fourth section of the act of Congress approved on the thirteenth of July, eighteen hundred and sixty- one; entitled "An act further to provide for the collection of duties on imports, and for other purposes," the President, for the reasons therein set forth, is authorized to close certain ports of entry:
Now, therefore, be it known that I, Abraham Lincoln, President of the United States, do hereby proclaim that the ports of Richmond, Tappahannock, Cherrystone, Yorktown, and Petersburg, in Virginia; of Camden (Elizabeth City), Edenton, Plymouth, Washington, New Berne, Ocracoke, and Wilmington, in North Carolina; of Charleston, Georgetown, and Beaufort, in South Carolina; of Savannah, Saint Mary's, and Brunswick (Darien), in Georgia; of Mobile, in Alabama; of Pearl River (Shieldsborough), Natchez, and Vicksburg, in Mississippi; of Saint Augustine, Key West, Saint Mark's (Port Leon), Saint John's (Jacksonville), and Apalachicola, in Florida; of Teche (Franklin), in Louisiana; of Galveston, La Salle, Brazos de Santiago (Point Isabel), and Brownsville, in Texas, are hereby closed, and all right of importation, warehousing, and other privileges shall, in respect to the ports aforesaid, cease, until they shall have again been opened by order of the President; and if, whole said ports are so closed, any ship or vessel from beyond the United States, or having on board any articles subject to duties, furniture, and cargo, shall be forfeited to the United States.
In witness whereof I have hereunto set my hand and caused the seal of the United States to be affixed.
Done at the city of Washington this eleventh day of April, in the year of our Lord one thousand eight hundred and sixty-five, and of the Independence of the United States of America the eighty- ninth.
[L. S.]
ABRAHAM LINCOLN.
By the President:
WILLIAM H. SEWARD,
Secretary of State.
The Constitution forbids it to the United States, however. Article I, Section 9.