Tiny question that requires a large answer:
Have you seen a doctor yet? If not, get in there. If you have, what direction are you headed?
Sadly enough, there are so many different medical problems it might be, if I listed them all, you'd probably drive to my house and punch me in the nose, so instead I'm just going to advise, you've got to keep working the problem till you learn what the problem really is.
Nerve damage, circulation type diseases, Fibro, RSD, restless leg, and the list goes on and on. I too have a black leg (use to have both, but we chopped off the left one a year ago) all the way to the knee so I do understand what black is, but I would never dare diagnose a problem like yours. Do take it seriously though, and don't stop looking for answers. Keep very close track of how much of your foot is discolored. Has it spread or has it stayed the same?
Take care and good luck.