I'm thinking that once a Blockade is announced, most Freighters will avoid the planet to keep from getting destroyed.
As for implementing a Blockade, I'm doing it in my current game with a single ship - not a Transport, and no Troop pods, so I'm not sure what's going on there.
I will try it again; but every time I tried with a huge task force , it would not let me... and then my tiny troop transport did blockade it. I will have to try it again...
Remember that the evil which is now in the world will become yet more powerful, and that it is not evil which conquers evil, but only love -- Olga Romanov.