Rummy Nose Tetra black stomach

Does anyone know why my Rummy Nose Tetra has a black stomach? He arrived that way. They were all real skinny and white when I got them, but their colors have returned and they’re bulking up. He’s doing fine eating and schooling with the others. I’ve had him for about 3 months. Tank parameters are good. I do weekly water changes. Please see attached photo. Thanks!

Need a picture that is in focus.

Has it had the black mark since you got it?
Did it have the black mark in the bag when you first got it?