Unfortunately mud also comes in many colors. However it is black stool that is concerning in that it could be digested blood that is getting into the upper part of the colon. Of course bright red boold also is a concern but is usually hemerroids. If you have bright blood and 'roid creams or supossitories don't clear it up off to the doc you go.
Chocolate can contain a lot of dye and dyes can make it through unchanged so it could probably darken stool towards brown. A prime example of dye that makes it through is the natural coloring of beets which can cause concern if you look back to see a toilet bowl full of bright red stool!!!!! Also licorice dyes(black, red or green) can get through, and of course if you binge on any of those you have a better chance of seeing the change.