Is it permissible to use black henna, knowing that many doctors think that it is harmful to a woman or to her unborn child?.
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.
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.