What are the effects of using different types of ground mustard? I made a cheese souffle recipe that called for 1 tsp of dry mustard. I used ground mustard that was 40% yellow and 60% black.(And it wasn't fine like powder)  Everything went perfectly with the souffle but the taste wasn't all that amazing. In fact, I couldn't stand eating it, perhaps because of the mustard spice I used, it just didn't taste good. I even reduced the amount of dry mustard to ~90% of 1 tsp, because I feared that its pungent taste would overcome everything else. So, now I am left wondering where did I go wrong? Is there a difference in taste with my described mustard and 100% yellow mustard? Would this difference cause such a huge taste differences?

I used Alton Brown's recipe: http://www.foodnetwork.com/recipes/alton-brown/cheese-souffle-recipe.html.

The only change I made is add a pinch of nutmeg and cayenne pepper instead of the garlic powder.
Brown/black mustard seed is hotter and more intense than yellow. So, powdered mustard seed that is predominantly brown/black is going to pack quite a wallop compared to powdered yellow mustard seed. In the US, recipes calling for powdered mustard are going to expect you to use yellow. See Serious Eats for an interesting article on the subject.