Black staining on teeth

[KAYAY]

I have this black recurring staining on my teeth (photo attached). It only appears to happen on the teeth that are directly exposed when I smile or talk. It happens on both the outer and inner surface of the teeth. It comes off when I aggressively use baking soda and scrub it off but that causes my gums to bleed. I do not use any medications that would interfere with it nor do I take iron supplements or drink any coffee or tea. The dentist can't figure out what it could be. It's been present for about 5 years now. I've lived in many different houses so it isn't the water. Does anyone have any ideas..? It's really frustrating and embarrassing and irritates my gums to have to scrub it every week or so.