Will this gap/black triangle between my teeth go away on its own? (Photo)

the black triangles usually don't resolve, in another words, gums don't grow back. if it truly was due to trauma and irritation, it may refill but you'll know in about a week or so after it happened. if it's still there, you may have to go back to your dentist and have them bulk up the filling a bit to fill the space. injection of dermal fillers also help fill in the gap although temporary. 

Depending on whether the contact point of
where the filing was, was moved further towards the edge of the tooth will
determine if this will reduce the black triangle. The fact that you’ve
irritated the gum can sometimes result in a little bit of a retraction of the
gum. I guess the real answer would be to wait a month or so and see if the gum
moves back down. If not, I would recommend addressing that gap by placing a
filling a little bit closer to reduce it. The problem is if a contact point or
where the teeth touch is less than five millimeters from the height of the bone
below the gum, then generally you don’t get these black triangles. If it’s
greater than that and it’s being moved beyond the five millimeters, you’ll find
you don’t get an infill of that pink gum area.