Henna alone will only add a red hue to hair, the amount depending on how light or dark your hair is. If your hair is black or dark brown already, you would probably only get a red shine in bright light, with no color change in normal light. The lighter your hair, the more red you'll see. The explanation that worked best for me was that henna was like red water color. On white paper, it shows up red, but on black paper it doesnt show up at all.
If you are looking for black, you would have to mix in indigo or use indigo alone(I'm not entirely sure you can do this as I've never wanted to dye my hair black, just red). Henna for Hair is a website with a LOT (seriously a ton of info) including a downloadable ebook with information about henna, indigo, and other natural hair dyes and how to use them. There is also a forum you can visit to post questions or get other advice.
I would definitely do a lot of research before taking the henna/indigo plunge as henna is permanent, as in the only way to get rid of it is to cut it. Some have had sucess lightening it, or using a color remover to get rid of it, but it took many many uses to remove all of the color.
Also if you have previously dyed hair you will need to use body art quality henna, henna of lesser quality can have severely damaging effects when mixed with hair dye (meaning your hair could melt, break, fall out or worse). Also from what I've read indigo can look green if not applied properly, so be sure to do your research. Here is the link for Henna for Hair:
Henna for Hair
Good luck if you go this route, henna itself is awesome for hair, I can not speak for indigo as I have not used it.