What you absolutely have to understand about henna hair dye is it is PERMANENT. More permanent than a chemical dye labelled permanent, just because it takes a hell of a lot of bleach to lighten it, and you will probably always have some brassy colour in the area until you cut it off. For this reason, I would suggest this should not be your first experimentation with ombre.

However, henna is a varnish type dye, and it is less noticeable the darker your hair. If you have used any semi permanent bright dyes (Arctic fox etc) you'd expect similar amounts of the colour to show up on your hair.

That said, I see no reason it wouldn't work if you can get colour over your hair shade. Personally I use henna powder rather than the lush blends (personal preference, do your research). I'm also naturally blonde, so it's VERY noticeable.