Henna is a UV blocker. What it does on wood depends on how it's prepared, applied and the type of Henna used as well as its age and post-harvest storage environment. Often the difference between high-quality and $#@* is the packaging & storage, I've gotten the same brand from different wholesalers, only difference was the storage environment ... lessons learned the hard (expensive) way, now I only deal with one reliable source. On exposure to UV, it may darken or lighten somewhat, most will darken but again it depends on the combination variables. The reason it's temporary on skin is because the stain is on the skin cells and as the skin sheds, the pigment goes with it.
FYI, there is no such thing as: black, negro, cobalt, indigo, jade, green, blau or any other color besides the reddish rust/brown. Black is extremely dangerous as it is often made with toxic chemicals that are not only absorption hazards but can cause severe chemical burns on living tissue - all colors other than natural should be considered hazardous.