Unfortunately, for Black Sea ports, that's true. The North Sea acts in the same way, as does much of Iberia. Other cities seem to use other routes when one becomes unavailable. There are three great trade nodes in the game...Salamis for the Eastern Med, Crete for the Central Med, and Caralis for the Western Med. Later in a game, you will see four or even more trade routes that run through these port cities. They aren't all being used, so it seems others open up when certain ones become closed. Blockades become especially hurtful if you close any of those ports, because certain ports get bypassed due to not having anywhere to trade.