I have a really awesome recipe for Indian fish that calls for mustard seeds, without specifying what color, so I always use yellow. I have never noticed any bitter flavor in that dish. Maybe you could use yellow instead of black? Don't give up on mustard seeds!
When you start to cook, as when you begin to live, you think that the point is to improve the technique until you end up with something perfect, and that the reason you haven’t been able to break the cycle of desire and disillusion is that you haven’t yet mastered the rules. Then you grow up, and you learn that that’s the game.
Adam Gopnik, The New Yorker