My 6yo (allergic to milk, raw eggs and tree nuts) is getting dark stains on her teeth.

Brushing (electric toothbrush, Colgate 2-in-1 for kids) does not help.  The dentist has no idea what causes it and my other child (3yo, not allergic but eats mostly the same food except an occasional yoghurt) does not suffer from this phenomenon.  There are no iron supplements, no excess chocolate, etc.  We're at a loss.

Does anyone have an idea what could cause something like that?