If a nation declares a blockade it is an act of war against another power and we were claiming that the South was still in the Union.
It was like declaring war on oneself.
The southron contingent love to trot that out as sort of a left-handed recognition of confederate sovereignty by the Lincoln Administration, but I'm not aware of any international treaty or Constitutional clause that prevents it. Nor am I aware of anything that equates a blockade of one's own borders with war. We weren't blockading France after all. And are we not in effect blockading our own borders today by throwing out drug interdiction screens and border patrols?