If tis completely blue and longish, it sounds like a Buccochromis spectabilis. If its blue with a higher body shape it might be a Heterotaenia(these are extremely rare in the hobby due to their size)
The other species don't get really blue. The Nototaenia gets more orange, the Rhoadesii yellow with a blue head(both these species have a higher build as well, with the nototaenia having a uhm flat head(as if it swam into the glas of the aqaurium))
The Lepturus gets more greenish and has an elongated form.
Expected sizes of these different species in an aquarium:
Nototaenia -- This is the smallest at (8-10")
Spectabilis -- It is said that these species stay smallish(didn't keep these untill now) I believe 10-12" would be a good estimate of the hear say over here
Rhoadesi -- These can get fairly large 14" is no exception they get a lot of body as well so 14" is quite a lot of fish
Lepturus -- These get a little longer 14"-15" than the Rhoadesi but they don't get so large in body mass as they are more elongated
Heterotaenia -- This is the heaviest endemic Malawi cichlid(only the Serranochromis Robustus(which is non-endemic) exceeds the body weight of this fish) A grown Individual easily reaches 16" with a real high body form.