henna & hair relaxers

I think all chemical relaxers have to be pretty harsh chemicals to work.  I think your only hair safe method would be to set it wet on super large rollers to help straighten it out, and see if that helps any.

As for henna, if your hair is black, you aren't going to get any lighter colors, but say if you use a red shade, you would get black hair with a dark red tint to it in the sun.  (Henna doesn't lift).
Real henna (body art quality) is permanent.  The henna you get at most stores has metals in it and isn't permanent.  Here's a site with some info on that showing many people's mixes with BAQ henna:
http://www.hennaforhair.com/mixes/index.html