Black rings around ankle/neck skin error.

That exact same thing happened to a close friend of mine. It was around his ankles and neck, reminding me of chains... I told him maybe he was a slave and didn't know it.

Eventually it did go away, I think he fixed it by rebaking, not sure.

Edited to add: It also might have been a bad "bald" hair. Try changing that too.

A Long time ago I used to have problems with various parts of my avatar turning black. It was caused back then by the fact that SL did not like ATI cards at all then (yes ATI and SL get along much better these days).

The only thing that would correct the issue for me (without replacing the video card) was to go into appearance and move one of the sliders a tiny bit. Might be worth a try.

I've had this problem several times and I've found that, if I drop on my avi an entire folder containing a "make outfit" complete with shape, skin, etc., it removes the rings which, for me, were around the neck and the waist.

So, hopefully, at one point in the past, you saved an entire outfit.

Good luck!