Slitherine

I have Rome under sea blockade. However, in the pic, Rome is still trading by sea. They even have land trade to another blockaded harbor that is trading by sea with a region under siege. I know that the region under blockade loses the income from the harbor, but shouldn't that cripple sea trade? It makes the blockade seem a complete waste. I need three ships to maintain a continuous blockade, so I am almost losing more in cost than the Romans.

AFAIK, blockade in the game only affects supplies in seiges and has nothing to do with trade. That is what the manual indicates.

But it seems like a good idea to investigate after the diplomacy update comes out of beta. On the other hand, I'm not convinced that "blockade" of trade would be in any way historical or even practicable in that era. Maybe something closer to raiding against trade would fit; i.e., a chance to diminish part of a port's trade.

Blockading an harbor will prevent its use for maritime trade. Trade might still happen if the region is not under siege.

It can also impeded your enemies from landing on your shores.