Dimidiochromis compressiceps
I knew of these... but I have never seen them actually be blue. I think its just me though.
Just from some Google searching...
Aristochromis Christyi 12"?
Champsochromis Caeruleus 18"?
Tyrannochromis Nigriventer 14"?
Tyrannochromis Macrostoma 14"?
I have no idea how readily available these fish are though.