Ruling on using black henna
Question

                Is it permissible to use black henna, knowing that many doctors think that it is harmful to a woman or to her unborn child?.
Praise be to Allah.
      The basic principle with regard to women using henna is 
      that it is permissible, because it is one of their adornments. For a 
      discussion on that please see the answer to question no.
      105417. 

    If there is a type of henna or any other product that is used 
    for beautification and adornment that is medically proven to be harmful to 
    health, it is haraam to use it because of that, because if it is proven that 
    anything is harmful, then it is established that it is haraam. 

    See the answer to question no.
    89719 and
    26799. 

    If it is not proven to be harmful, then we go back to the 
    basic principle which is that it is permissible, and we have not come across 
    any scientific study that reliably proves that black henna is harmful to 
    health such that we could rule that it is haraam. Reference should be made 
    concerning that to specialists, and if it is proven that it is harmful, one 
    should act upon that and keep away from it. 

    And Allah knows best.