Big Black Butt Hole - Barbs

[milkton]

Hi, can you guys help me figure out if this is normal?

Well, my main food for my barbs (albino, tiger, black ruby, green tiger, and denisoni) are bloodworms. I know bloodworms are supposed to be a treat but they dont seem to like the flake food.

Anyways, i was looking at them a few days ago and noticed they're big blackish-brown butts, right underneath their stomach. Some of them have it, and some of them don't.

Is this a disease? Is it normal? Please help!