Henna tattoo come off in a week or may be two to three weeks. They don’t stay forever. They are not permanent tattoos, they are temporary tattoos. But yes if you use black henna and get allergies then there are chances that it will leave impressions or marks caused by skin burning so please be aware of using black henna.
Please take an allergy test by making a small design before using black Mehndi to check that whether this Mehndi suits you.