henna (mehendi) on black hair?

No effect.

Henna only works on light hair. That's why your mom used henna

to COVER her white hair.

If your hair is jet black - not a good chance

if its lighter then there is a slight chance you could see a faint highlight.

But no, just use normal dye

Indoors, it will look shinier, but you won't see any red until you're out in the sun. With the sun behind you it will have a red sheen, and it may look more black-cherry than just black. If you have grey hairs, you will have red strands instead. Henna gives great shine and smooths hair, so even if you don't really see the red it will still look healthier and better. If your hair has a significant number of brown highlights, either from the sun or just genetics, the red will be more noticeable than on your darker hairs.

i got mehendi put on my hair at the parlour. i leave it on for 3 hours and then just rinse with water. do not use shampoo. its very messy while washing it off. wash it again the next day with shampoo and then ul finally have clean hair. you get a slight reddish tint to ur hair. it looks really good but goes away in about a month's time. any greys will turn dark red in colour. so if u wanna hide ur grey hair, mehendi is the best option. however the down side for me is that i have straightened my hair because of which iv been told that i cant use mehendi as it will make my hair rough. so now im stuck with all my grey hair looking very grey!!!

personally.. i think highlights on dark hair such as black... looks tacky and wrong, makes it loook like people dont know what theyre doing and it dosent look good. i would die it like a medium brown first and then add highlights... (all over)

yeah. if you put red dye on your black hair, it will turn maroonish

its not gonna werk