You Can Use Natural Henna
Natural henna, which is a reddish-orange tone, is safe for use over colored, bleached or permed hair. You must first do a strand test to ensure that your hair won't turn an awful green or black color first.
Natural henna only comes in the natural red-orange color, and any other color contains other ingredients, including possible chemicals, to get a different non-red color.
Don't Use Compound Henna
Compound henna is henna that has chemicals or metallic salts added to it to give it a different color. Compound henna should not be used over recently chemically dyed hair for at least 12 months, and then only with a strand test to ensure proper coloring and that your hair won't "fry."
Compound henna put over chemical hair coloring can completely ruin your hair.
Most "black" henna products contain PPD (Para-Phenylenediamine), which can cause skin irritation and allergic reactions.
Only use natural henna and do a strand test. Clip strands of hair from your head and apply the henna to the strand, leave it for two hours and wash it off. Dry it to see the coloring.
Ensure you've read the ingredients on the box. Purchase your henna from a reputable dealer.