Black Henna for hair?

I'd like to take my dark brown hair a shade further to black. In the past i've used regular black hair dyes which I was never completely contented with. A friend suggeted I try out black henna - also to strenghten my hair as it has began thinning from to many chemical hair treatments.

What are the positive results for the hair when using henna? How is it made and used, and where can I purchase it?

Black Henna for hair?

http://www.wikihow.com/Dye-Your-Hair-Dar...

http://www.wikihow.com/Dye-Your-Hair-Wit...

http://www.wikihow.com/Apply-Henna-to-Ha...