The triangular piece of gum tissue that normally fills an area where a black triangle exists is called a papilla. The papilla can shrink and recede in the presence of periodontal disease (bone loss). Once the underlying bony support is no longer there, then the papilla no longer has enough support to occupy the space between the teeth and recedes, leaving behind a black triangle. Teeth that are more tapered in shape with narrower necks tend to be more susceptible to black triangles forming when gums recede. Sometimes, when such shaped teeth are very crowded and overlapped prior to orthodontic treatment, there was never much space for any papillae between the teeth to begin with. Then, when the teeth are aligned and no longer overlapping, black triangles become apparent but it's not because the gums receded, but rather that they were never there to begin with. Usually, orthodontists will slenderize the teeth in a process called interproximal reduction (IPR) in order to make the teeth shape less tapered and then bringing them closer together to reduce the size of the black triangle. Another option is the injection of a hyaluronic acid filler directly into the gum tissue in order to plump up the papilla, but this is not permanent. Other than that, your gums will not ever fill up your black triangles, unfortunately.
