Is it permissible to use black kohl on the chin to conceal a deformity in the mandible resulting from an accident?

There is no harm in applying black kohl in the manner described, as it is not dyeing the hair black, which is a matter of scholarly disagreement. We advise you to be content with Allah's decree and to look at those who are in a worse condition than you. For a person's perfection lies in their religion, intellect, and character, not in their appearance.