Well, I've heard it told as a three-legged stool, like this one: http://www.taunton.com/finewoodworking/Gallery/GalleryImage.aspx?id=22866
This riddle originated in the Appalachian mountains (North Carolina) but is now considered fairly racist. Riddles like these with sexual overtones, but with a pretty innocuous answer were popular back then.
I found this on a site that catalogued an archeological dig in North Carolina:
"Another method of cooking was to set a pot or pan on a three-legged stand over hot coals."
"Black upon black, black upon brown, three legs up, six legs down, what am I?"
I heard this many years ago from my grandpa, and the answer was a black man with an old cooking pot on his head (legs of the pot upward, wearing it like a hat), and riding a brown horse.
There used to be black cast iron cooking pots that had three legs and a handle. They were often hung over the fire by their handle. This is the black upon black - the pot on the black man's head. The pots looked like those here: http://images.google.com/images?hl=en&rls=WZPA,WZPA:2005-35,WZPA:en&q=black+cast+iron+cooking+pot+3+legs&um=1&ie=UTF-8&sa=N&tab=wi (the third one, top row is the most like the one in the riddle)
The black man on a brown horse is the black on brown. (Another version says, "black upon black and brown upon brown..." and this was supposed to be a black man in brown breeches (pants), so brown upon brown.
The numbers come from the horse = 4 legs and the man = 2 is 6 legs down. Then the old cooking pot = 3 legs upThis was, and I imagine still is, considered a racist riddle.
I hope this answers your question, and thanks for trusting Just Answer!