A good trader will divert his fleets the second he or she gets word of a blockade. Large traders will have more than one source of trade income. Attacker will commit ships and defender will wait it out - win or lose, the trade is already gone.
Attacker will then have to decide to continue or move on. Defender has waited three, four days. May decide to wait again.
Blockades may be premature - if port control is to be defended with the parameters used here, there has to be much more value to port control. Taking influence, population has been done, and can still be done with voodoo cards. I imagine that is where that battle will be fought, as adding population by ships is a slow process.
The true battle will take place after the blockade when ships hit the plunder screens, where the efficient use of a few battle fleets and voodoo may occur, if the rewards of the action is justified, or goals can be reached.
Finally, I don't get the idea that one goes into a massive battle and expecting to NOT lose ships, be you a defender or attacker. It is not realistic at all.