All perfect praise be to Allah the Lord of the Worlds. May His peace and blessings be upon our Prophet Mohammad and upon all his family and companions.
According to the reliable opinion of our School of jurisprudence, Shafie, it isn`t permissible for a man to dye his hair black based on the sound narrations mentioned in this regard. An-Nawawi (May Allah be pleased with him) says: "The correct opinion is that this isn`t permissible. He stated in the end of [Al-Ahkam Al-Sultaniah] : "The Islamic inspector of the market or administrative supervisor prevents the people from dyeing their hair black except the Mujahid (One who fights in the way of Allah)".
The evidence on this is reflected in the following narration: "Jabir told that when Abu Quhafa (Abu Bakr’s father) was brought on the day of the Conquest of Mecca with his head and beard white like hyssop, the Prophet said: “Change this with something, but avoid black.” {Related by Muslim}.
In addition, It was narrated that Ibn 'Abbas, who attributed it to the Prophet [SAW], said: "Some people will dye their hair black like the breasts of pigeons at the end of time, but they will not even smell the fragrance of Paradise." {Related by Nasa`ei, Tirmithi and others}. {Al-Majmou`, vol.1/pp.345}.
However, the majority of the Hanafie, Maliki and Hanbali jurors are of the view that this is reprehensible, but haven’t asserted that it isn`t permissible. Some of them even considered it permissible based on some Prophetic traditions indicating that Al-Hasan, Al-Hussein and others used to dye their hair black. This is as stated in [Al-Musannaf" of Ibn Abi Shaibah,vol.6:/pp.2].
In [Rawdat At-Talibeen] An-Nawawi (May Allah have mercy on him) excluded dyeing the hair black to beautify herself for her husband. According to him, this is permissible because it achieves a benefit and wards off an evil. This same view was adopted by Al-Ramli and others. Kindly check [Tohfat Al-Mohtaaj, vol.9/pp.376.]
Accordingly, in principle, we see no harm in selling and trading in black dye because it has permissible uses, in addition to the possibility of mixing it with other dyes to create shades of other colors. However, if the seller learns that the man buying the black dye intends to apply it on his hair, then it isn`t permissible to sell it to him. And Allah the Almighty Knows Best.