You probably shouldn't, but we won't tell!

Depending on where you live black mustard seeds can be kind of hard to find. There is a subtle difference in flavor but if you need to substitute go ahead.

Black mustard seeds are stronger than yellow, so you may want to increase the amount of yellow when you substitute

I just tasted a yellow mustard seed and a black mustaard seed. The yellow is milder and sweeter than the black, which I've used to make mustard. Overall, I would stick to the yellow seed.