I've searched the forums and found no real mention of this. It would make more sense/be more useful if the basic "defence" of the planet had to be overcome to even form a blockade. This would prevent the silly scout blockades, but generally only require 2 typical battle ships to perform. You could technically even add a support module for ships that helps you build smaller faster "blockade" ships (i.e. increase influence while orbiting) so if people want to "harass" they still can, with a bit more effort.