She dyed her hair black without knowing that it is haraam. Should she leave it or change it?

Firstly: 

If there are any grey hairs, then dyeing them black is
haraam, but if the hair is already black, there is nothing wrong with dyeing
it black. 

Shaykh Ibn Baaz (may Allaah have mercy on him) was asked:
Some women use a mixture to condition their hair. This mixture includes
henna and other herbs, one of which dyes the hair black. What is the ruling
on using this mixture? Please note that they use it for the purpose of
conditioning their hair, not for the purpose of dyeing it black, because the
hair of some of them is black. 

He replied: 

There is nothing wrong with using the mixture mentioned to
condition the hair if the woman who is using it does not have any grey
hairs. But if there are any grey hairs, it is haraam to use anything that
will make the grey hairs black, because the Prophet (peace and
blessings of Allaah be upon him) said: “Change this grey hair, but avoid
black.” End quote.

See also the answers to questions no.
476,
47652 and
1187. 

Whoever does that – whether male or female – knowingly and
deliberately, has fallen into sin and has to repent, seek forgiveness,
regret what has been done and resolve not to do it again. Part of repenting
completely is removing the black with something that will not cause any
harm. 

But if a person was unaware of the ruling, there is no sin on
him, but he has to change it. He is excused for it but there is no excuse
for leaving it as it is. 

An example of that is tattooing. If a person was tattooed
when he was young, or when he did not know that it is haraam, then he must
remove it when he comes to know that it is haraam, so long as removing it
will not cause him any harm. 

Shaykh Muhammad ibn Saalih al-‘Uthaymeen (may Allaah have
mercy on him) was asked: 

What is the ruling on tattooing? If a girl was given a tattoo
when she was young, is there any sin on her? 

He replied: 

Tattooing is haraam, and is a major sin, because the Prophet
(peace and blessings of Allaah be upon him) said: “The woman who does
tattoos and the woman who has a tattoo done are cursed.” If a girl is given
a tattoo when she is young, and she cannot prevent that being done to her,
then there is no sin on her, rather the sin is on the one who did that to
her, because Allaah does not burden any soul beyond its scope, and this girl
had no control over the situation. But she should remove it if she is able
to without that causing her any harm. End quote. 

As’ilah tuhimm al-Mar’at al-Muslimah
(question no. 20) 

Secondly: 

With regard to the
ruling on removing the hair that is between the eyebrows, in the answer to
question no. 21400 we have quoted the scholars of the Standing Committee as
stating that it is not part of the eyebrows, and based on that it is
permissible to remove it.

 And Allaah knows best.