Sonofabitch, I had a huge explanation. I'm not gonna type the whole thing up again.
Basically, it is slang, means the reflection of light off of something. For example, someones 20 inch chrome rims would be called "bling", because of the reflection of light coming from the chrome. Another example would be to say that a Rolex is "bling" because of the light refracted by the diamonds. Since shiny things and money are equated with each other (silver, rolexes, 20's), it is also a sign of prosperity and money. For instance, if i was holding a bunch of 100's, that would be bling blingin. Therefore, rich people, or people with money or who just bought something, would also be bling bling.