These are some of our metriaclima sp. blue dolphin manda....aka giant demasoni. The males get about 7" in length and a little darker when not excited. I'm not saying this is what it is, it just looks similar.
Noki has a good suggestion also, some type of cynotilapia.
So many look a likes. This is why buying fish with no name and trying to identify them later is just about impossible.
Here is a pic of a female manda strutting her stuff.
The Cynotilapia has the BB and black dorsal, while the Metriaclima BB Zebra has a light blue dorsal, and the Elongatus has a yellow tail. Those differences are how they can tell each other apart in nature. If you don't know where the fish come from it gets very confusing to ID.