How Much IPR Can Be Done to One Tooth?

Although your question doesn't indicate whether the "black triangle" was on the upper or lower, I assume that the upper front teeth is the problem area. If you are saying that .5mm was removed on each tooth, that seems more than is typical of Invisalign cases.  However, IPR can be done safely to remove the majority of the enamel (as there still is some enamel remaining for protection) which would be at least 2mm thick at the contact area of each of the upper central incisors.  So doubling the IPR of .5 mm still should be no problem.  Other questions remain in terms of esthetics, relating to the size and shape of the central incisor teeth as well as the adjacent teeth to create the best result, however your question of the safety of additional IPR is at least answered.  

Your Dr best knows how much can be taken off. The enamel is thick and .5mm is not much. It seems a consultation is needed so you can feel comfortable with your treatment.

its your teeth so ask questions

The black triangle space that you have is because you have 2 triangular teeth sitting close to each other and only contacting at the tip. The bone and soft tissue is also missing due to the initial crowding.

Whether you have 1-2 mm of enamel, it is not advisable to do more than 1mm IPR between 2 teeth. This allows the triangular contact points to be separated and the teeth brought closer together. The space may never be closed unless you want rectangular teeth as a result.

Too close and the roots may impinge on the bony separation and cause further gum shrinkage.

I never do more than 0.5mm  IPR in most cases and only when the teeth are nearly straight or there is a blindingly obvious big space present.

Your problem is not solved with IPR the length of the tooth, but only the half close to the biting edge.

send me a picture and I will advise