CSA taking Michigan and Illinois cities on lakes, or California ports don´t change negative effect of blockade.
In my last game as CSA, I have invaded California, but no change in my economy (ok, to transport cotton in the desert may be difficoult ... but I could receive weapon or any help from abroad).
Then I have occupies all USA port on great lakes, opening comunications with english Canada, and no change in my economy (cotton can be trasported by rail in to Canada).
I think that the effect of blockade should stop occuping California or Lakes ports.