I've noticed recently a black spot has formed, beginning as a stain, on the right side of the nail plate on my left big toe.

Now there's two more of them, one below the original near the curvature at the base of the plate, and a third on the opposite side, also in the curved area near the base.

The last major trauma to that plate was not a crushing event, but me catching the free edge on something and tearing it off. My dog (large dog, 70 pounds, extremely clumsy) steps on my toes quite a bit, but all the other nail plates show no signs of damage.

As I recall, the affected nail plate felt loose before this (it really feels loose now, as whatever's happening is happening). Did some nasty thing get up under there and light up an infection?