I'm aware this might not be a big deal at all, but I just want to know 'cause I have zero medical knowledge:
So here's my right foot: http://imgur.com/HCXRz2R
That black area wasn't so big months ago, like it spread just now.
And now it's happening in my left foot: http://imgur.com/amaYccR
My guess for months has been that's just coagulated blood. But, again, that's just a guess. So... Should I be worried?