You asked why the color black is used to represent a master's rank in most arts - here's what I read several years ago:
all students wore a white belt. over time, they would sweat, and the sweat made the belt yellowish. (Note: they never washed their belts.) as the training got harder, they were introduced to some type of grappling, or hit the ground in some manner - getting swept, knocked down, etc. and grass stains added a greenish tint. Training got more rigorous and the belt turned brown from a mixture of sweat, dirt and blood. eventually, the belts turned black.
Dunno how much truth it holds, that's just what I read.
As far as young black belts go, I don't think one should be given at a young age - I would not give one to anyone under 16 or 17, and they would have to be exceptional at that age. The reason being is that someone so young will not be able to use their skill properly, as far as power goes. a 9 year old black belt can't do squat to an adult with his hardest kick, unless it's in the knee or the nads, and you don't need a black belt to be able to do that.
i'm nobody...i'm nobody. i'm a tramp, a bum, a hobo... a boxcar and a jug of wine... but i'm a straight razor if you get to close to me.
-Charles Manson
I will punch, kick, choke, throw or joint manipulate any nationality equally without predjudice.
- Shonie Carter