That might have been me.

Just sites I got from others on this forum or stumbled across in my own quest to ID all the critters in our tank.
Nerites seem to come in several colors/patterns. Here's a site with lots of photos.
http://www.chucksaddiction.com/Hitchsnails.html
If you scroll down (about 1/3 of the page?), there's a photo of a black shell. We have a few in our tank, I think. It looks like one used to have black, ridged shell; another couple have smooth, patterned ones like at the site bb029 listed. Both kinds in our tank have a black 'foot'. The biggest looks like it's around 3/4 of an inch at its widest.
Hope you find the info you were looking for.