This happens when the gums in between your teeth have gone down a bit or receded. There are a lot of different reasons for gums to not fill all the way in between the teeth:
There's really no problem with having these black triangles but patients don't like how they look and food might get trapped so they can cause a plaque trap.
Sometimes, if teeth are very crowded and overlapping and you get them straightened with braces or Invisalign, black triangle spaces will appear that were not visible before. This means that there were some of the above problems but now you can see them.
Your orthodontist can fix these black triangles to some degree with some stripping of the teeth. It varies from patient to patient how much you can fix these. See Dr. Markova today so she can address your concerns.