This goes back to an earlier point in the discussion.
Essentially, Navy Secretary Gideon Welles campaigned earnestly for Lincoln to declare the ports of the South closed, as that was something that was clearly legal under existing precedent and did not have the weakness of an implied recognition of the Confederacy as a belligerent. But Britain would never have gone along with it, and attempts to coerce them into going along with it would likely have led to an increased chance of war with Britain. The blockade decision, though it had the "con" of implied recognition of belligerency, was something that Britain would tend to support; first, because it allowed her to remain neutral instead of forcing her to decide which side to support (as voluntarily abiding by the port closure would clearly be aiding the North); and second, because Britain herself was one of the most frequent operators of blockades, she had a national interest in increasing the legitimacy of blockade operations.
The actual proclamation, if one reads it, tried to have it both ways, giving the impossibility of collecting customs fees as the rationale for the blockade and declaring the ports both 'closed' and 'blockaded,' but this had no validity under existing international law.