Having “Blackstreet Blues” means to have a sort of sadness, in a way that isn’t seen fully. It combines the word “backstreet” (referencing how backstreets aren’t seen easily, and they need to be examined closer) and “blues” (aka, sad).
Person A: Hey man, I heard your dog just got ran over, but you seem to still be smiling. Are you okay?
Person B: Nah man, I got a case of the backstreet blues