See here (http://forums.civfanatics.com/showthread.php?t=301021)

I don't think it is a bug strictum sensum, but the double standart compared with the current blockade situation to other players ( you can't blockade another civ from your sea tiles ,for a example ) makes me wonder if this is the more correct implementation.....

Thanks rolo. I'll take a look at how that situation compares to both privateers and enemy units who are at war with both civs.

Okay, given that blockading barbarians ONLY occurs with the Better AI mod, I don't think it's a problem that should be addressed in the unofficial patch.

If the Better AI mod is going to introduce this kind of stuff, then it should fix the bugs it introduces.