I have burundi. I have raised them since they were one inch fry. I have seen this dark line that you are referring to. I have my tank in the living room where i can watch them for hours each day, i have noticed that the dark line comes and goes with mood and activity of the fish. All of the fish have this dark line, even as one inch fry up to the largest alpha. I have nice fish so i dont think it is from inbreeding, it seems to be more of a "body language" coloration for communication. Like when a alpha goes after a sub dominant fish, the subdominant fish goes all pale (loses all blue) , the black gets blacker and the dark line appears. For the alpha he gets the dark line when i rearrange the tank or swap a tank mate out and add a new one etc.. Personally i equate it with "unease" about something. Hope this helps.