The answer is - Yes! it's possible to henna all skin tones, henna comes out the same colour on all skin types but this does mean of course that there won't be as much contrast with henna on very dark skin.
This means that it's fairly important to pick the design carefully. Because there is less contrast you don't want to go for something that is really intricate, as the delicate lines may not stand out enough. Bold desings tend to work better on dark skin - this is reflected in the bold and striking henna designs of some of the African countries.
So to anyone with darker skin that is thinking of getting a henna tattoo - go ahead! henna can look amazing on dark skin, but give some consideration to the design. If you'd like to book an appointment with me I can help with this.