I'm not 100% sure that the trade interdiction has to be in the province. I'll double check tonight but I believe I had a port trading silk which is not in its province but instead its in the neighbors province. If that province's trade is disrupted then this port province trade income might also fall.

Also you get trade income for exports and imports. If your trading partner is experiencing trade interdictions, wouldn't that cause your trade numbers to go down? Just a thought to throw out there for experimentation.