I've never had a problem with a neoprene girth regardless of the color of the
horse. I just love them! But here are a coulple of things that you might
check.
Is the girth worn or does it have any irregularities or cracks in it?
Is the skin pinched up in the girth area and maybe you have a rub mark caused
by skin rubbing against wrinkled skin?
Or maybe, like you said, the girth might be too tight.
I believe the theory behind these girths is that they cause the horse to
sweat in the girth area and this sweat lubricates the area, thus reducing
friction. Is your horse sweating enough to do this? Maybe you could wet the
area before you ride. . .just a thought.
Good luck.
~Nora