Is it permissible to use a oil that blackens hair, and does it have the same ruling as dyeing hair black?

It has been authentically reported that the Prophet, peace and blessings be upon him, forbade changing grey hair with black dye. This includes dyeing, applying oils, and other similar actions. However, it is permissible to use black oil for treatment, as long as it does not involve anything forbidden or result in harm, because in such a case, it is for medicinal purposes, not for dyeing. As for dyeing hair with henna, it is encouraged, but if one dyes their hair black, it falls under the general prohibition.