Mustard seeds in Bombay potatoes
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Would depend on your standards for taste: what French mustard are you going to use? How many seeds correspond to a teaspoon of it (how concentrated is it).
I believe you could substitute one thing for another for sure, but you will never know what is the original taste and probably that doesn't matter.
If the recipe requires black mustard seeds, there is nothing else that has that flavor profile, especially if you are instructed to take the step of starting out by frying the seeds in a covered pan, in hot oil until they pop. Oh, my...one of my favorite cooking smells.
I know Madhur Jaffrey uses a lot of yellow mustard seeds in her recipes. Perhaps yours calls for yellow mustard seeds, in which case I would use a touch of Dijon for the heat. Most course grain mustards have enough vinegar in them that I think if you used enough mustard to notice the flavor of the mustard, you would also notice the vinegar.