Hi Lance,
I will give you two answers. You are free to pick either of them. It is my opinion both are correct.

1) A casino man who worked at the Nevada Club told me that painting the rims black signified WHICH parking lot your car was parked in.

2) The rims where painted black so that when placed in a rack of $100.00 chips no one could tell, upon taking inventory, the cage was short.

Best, Mr."P"