Hi Martin,welcome to discus addiction
Black bars can be a sign of stress bcz of bullying,water condition,bars can serve them as camoufluage to blend with a surrounding or a dark background,sometimes when in a breeding mood they display them as well or during feeding when fighting for a better piece.In the mean time few strains have permanent bars as a part of a pattern(Turquoise,Snakeskin,wild types)
HTH