All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger.
What is Islamically forbidden is to dye one’s hair pure black. The Prophet said: “Change this (the white color of the hair of Abu Quhaafah, Abu Bakr’s father) with something and avoid black.” [Muslim]
As regards any other color that resembles black color but it is not purely black, then this is permissible. The evidence about this is that Anas said: “Abu Bakr dyed his hair with henna and katam (a plant which produces a reddish-black dye), and ‘Umar dyed his hair with pure henna.” [Muslim]; pure henna means without adding to it anything else. The narration suggests that Abu Bakr was used to combine between the two, and katam is a plant from Yemen which produces a reddish-black color, while the henna gives a red color, so dying with both of them gives a color between black and red. It is also reported that the Prophet said: “The best things to use to change grey hair are henna and katam.” [Ahmad, Abu Daawood, At-Tirmithi, An-Nasaa’i and Ibn Maajah]
With regard to the second part of your question, please refer to Fataawa 81994, 89743 and 85382.
Allaah Knows best.