Last night I started cooking green beans and potatoes in the crock pot, which I covered first with aluminum foil and then the lid, which didn’t fit tightly. I set the crockpot to warm overnight, but when I checked it this morning some of the potatoes were blackened. Are they still safe to eat?

With a container that full, I’m sure that some of the cut potatoes in the center were cool for quite some time and likely oxidized. But beyond the black color, I would suggest tossing them out and starting over to be safe. Sealing a lid at low temperatures can be similar to canning vegetables and could cause a food safety issue. See these links about leaving out cooked potatoes overnight:
Food safety
Pre-cooking baked Idaho® potatoes
I froze my potatoes and they turned black