Ruling on using herbs to dye the hair that give it a deep black colour

Firstly: 

There is nothing wrong with using natural herbs to dye the
hair or for other permissible purposes, so long as there are no harmful
effects, because Allah, may He be exalted, says (interpretation of the
meaning): “He it is Who created for you all that is on earth” [al-Baqarah
2:29]. 

Secondly: 

It is not permissible to dye the hair black, for men or
women, young or old, because of the general meaning of the evidence which
indicates that that is not allowed. 

The scholars of the Standing Committee for Issuing Fatwas
were asked: I have seen some people using something to change the colour of
their hair, whether they make it black or red, and I have seen them using
something else to straighten curly hair. Are any of these things
permissible? Do young people come under the same ruling as old people? 

They replied: There is nothing wrong with changing the colour
of the hair to any colour except black or with using something to make curly
hair straight. The ruling is the same for young people as for old people.
That is so long as there are no harmful effects and the substance is taahir
(pure) and permissible. 

With regard to changing the colour to pure black, that is not
permissible for men or women, because the Prophet (blessings and peace of
Allah be upon him) said: “Change this grey hair, but avoid black.”

End quote from Fataawa al-Lajnah ad-Daa’imah, 5/168 

Shaykh Ibn ‘Uthaymeen (may Allah have mercy on him) said: If
the hair is dyed black, the Prophet (blessings and peace of Allah be upon
him) forbade that when he enjoined changing grey hair but avoiding black, as
he said: “Change this grey hair, but avoid black.” 

There is also a stern warning addressed to the one who does
that, which indicates that it is haraam to change the hair to black. As for
changing it to other colours, the basic principle is that that is
permissible, unless it is done in the style of disbelieving or immoral
women, in which case it is haraam from that point of view, because the
Prophet (blessings and peace of Allah be upon him) said: “Whoever imitates a
people is one of them.” Narrated by Abu Dawood, 4031; classed as saheeh by
al-Albaani in Irwa’ al-Ghaleel, 5/109.

End quote from Majmoo‘ Fataawa wa Rasaa’il Ibn ‘Uthaymeen,
11/120 

What is meant by the warning is the words of the Prophet
(blessings and peace of Allah be upon him): “At the end of time, some people
will dye their hair with black like the crops of pigeons. They will not even
smell the fragrance of Paradise.” Narrated by Abu Dawood, 4212; an-Nasaa’i,
8/138; classed as saheeh by al-Albaani in Saheeh al-Jaami‘, 8153. 

Thirdly: 

It is permissible to use the herbs mentioned to straighten
hair, but if they give a deep black colour, it is not permissible to use for
one who has some grey hairs. 

Shaykh Ibn Baaz (may Allah have mercy on him) was asked: Some
people use a mixture to straighten the hair; this mixture is composed of
henna and a number of other herbs, which includes a herb that dyes the hair
black. What is the ruling on using this mixture? Please note that they are
using it for the purpose of straightening the hair and not to dye it black,
because some of them have black hair. What is the ruling on using it for a
woman whose hair is black but there are some white hairs that are not due to
old age, and she is also using it for the purpose of straightening her hair?
Please advise us concerning that, may Allah reward you with good. 

He replied: 

There is nothing wrong with using the paste mentioned to
straighten the hair if the woman who is using it does not have any grey
hairs. But when there are grey hairs, it is not permissible to use anything
that will turn the grey hairs black, because the Prophet (blessings and
peace of Allah be upon him) said: “Change this grey hair, but avoid black.”

End quote from Fataawa ash-Shaykh Ibn Baaz, 10/63 

Based on that, if these herbs dye the hair black, and make it
deep black as you say, then you should not use them.

And Allah knows best.