What does it mean when a bearded dragons foot turns black?

It can mean different things depending on the environment. If he/she doesn't seem harmed and acts like it usually does, it could be shedding, emotions, or stress. When they shed, they turn darker or lighter before shedding. Try giving it a bath for at least 15 minutes. Take a small baby toothbrush or paintbrush and carefully wash their feet to help them shed.

If the lizard is acting strange; walking low to the ground, not eating, limping, or twitching limbs, it may be better to talk to a professional. If he/she seems normal and fine, it's probably nothing to worry about. You could talk to a pet store anyway, just to make sure. Just watch it for a couple of days and try and notice small things. Also, check their cage/aquarium and the accessories. Double check that they have enough lights/heat and food.

Goodluck