Black triangles are caused by four factors. 1) The height of the alveolar bone (bone beneath the gum tissues) and it's distance from the interproximal contact point of the teeth (where the teeth meet). 2) The morphology (i.e. shape ) of your tooth and 3) the actual health and amount of gum tissue you have between the teeth. 4) the distance between your roots.
So there you go, all four need to be in concordance for this to work. Your black triangles were no doubt CAUSED by the orthodontist's movements. And would be even worse if you had extractions cause now the roots are spaced too far apart. We need to wait for tooth cloning/biological solution for the problem.