Although "bling bling" can be a reference to a shiny or valuable object, originally the word was prison slang. "Bling Bling" was the catchphrase an inmate would use to warn other inmates that a prison guard was approaching. This was useful as a warning to either hide contraband or cease any punishable activity. In essence, the phrase is similar to "heads up", except obfuscated to make warning fellow inmates safer.