Is it possible for gums to reshape into black triangles that show up after Invisalign?

Dark triangles are caused when crooked teeth are straightened, or when the triangular space between teeth is to wide relative to the length of the triangle. Poor oral hygiene can contribute.
The solution is to reshape the teeth then pull them closer together.
Eliminating these triangles is pretty standard orthodontic care and should have been addressed during your treatment.
Some form of retreatment will be necessary to fix this problem. If you do not fix it, it usually gets worse over time due to plaque accumulation.

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.

Unfortunately your gums will not “grow” and refill the black triangle. Black triangles are often caused by an issue with the shape of the teeth and how they contact each other, as well as the teeth being moved out of the bone/gums due to flaring or extruding the teeth during orthodontic treatment to gain alignment. Age also plays a role in all of this, which could explain why you didn’t have an issue your first go around with orthodontics. Often times to correct black triangles we will file or reshape the teeth to make the contacts more flat, thus reducing the amount of space under the contact.