Wrong. The question of the legality of the blockade was decided by the Supreme Court in 1863 in what are commonly called the "Prize Cases". The Supreme Court decided that the blockade was a legal extension of the President's powers to supress rebellion.
That is my major complaint with Adams' book. It if very poorly researched and full of inaccuracies such as these. The Civil War is one of the most thoroughly documented wars of the 19th century. There are warehouses full of government documents before, during, and after the war for both North and south. Every major figure in the war except Lincoln wrote memoirs. Every political leader of both the North and the south left masses of written documentation on their thoughts and positions both before the war and after. And yet Adams uses none of this. He quotes endlessly from newspaper editorials and nothing from the figures who actually made the history. All in all it is a very poorly researched work and little more than page after page of his own opinions.
BTW, I have a used copy of the book I am willing to sell cheap.
"That is my major complaint with Adams' book. It if very poorly researched and full of inaccuracies such as these"
Well, you major complaint is not founded on fact. That is not a quote from the author of the book.