Is it permissible to use Amla oil to soften hair, knowing that it blackens hair, and that this contradicts the Hadith: "Change white hair but avoid blackening it"?

If the oil is used to blacken the hair, then it is not permissible, because blackening the hair is not permissible. However, if it is used to straighten curly hair, and to increase its beauty and blackness, then there is no objection to it, as long as it does not involve something forbidden or result in harm from its use.