Did you know that blockading of ports is and has been recognized as an act of war?
Sure, but seizing dozens of United States forts, armories, arsenals, ships, mints, customs houses, etc., are also acts of rebellion and insurrection, if not outright war.
And the Confederate military operation against Fort Sumter on April 12, 1861 was an absolute act of war, to which Lincoln's response of April 15 was mildest possible.
Also remember that Lincoln ordered that blockade on April 19 in response to Davis' April 17 moves to issue letters of marque and reprisal, setting up a privateer naval force.
So my key point in all of this is that, on every step of the way from peace in 1860 to war in 1861, the Confederacy was always as step ahead, the Union simply followed, slowly, less effectively.