"So why do the Bedouin wear black in the desert?"
Not all wear black, some wear lighter colors. Perhaps it's more a matter of social/tribal distinction, rather than sheer practicality?
Humans in general are not known for sheer, or holistic practicality in all things we do or don't do. You can see this in many different cultures in many different ways.
Our western culture is filled with such examples, like high heels for women. Why on Source's earth, would any one subject themselves to such a convention that causes damage and discomfort to the body? Not practical in the least bit.
What i do know is that, there has been a HUGE difference when i've worn very dark and very light colors in the Sun during a hot day. Perhaps you would feel it less with more insulation underneath, but no thanks either way. What i could see as being more practical in the desert is to wear a thin, breathable black layer next to the body out of cotton that hugs same, and then over same some white or light colored, loose, flowing linen clothes. This would provide a lot of Solar UV and heat protection, as well as minimize moisture loss.
Many modern day Islamic men whose ancestors come from more desert like conditions, tend to wear a lot of linen or cotton that is long, flowing and loose, and often lighter colored. Just check out one of these online clothing stores that cater solely to people of the Islamic faith.
Linen use to be popular among ancient Egyptians, as well as ancient Hebrews. Sure the latter like other Middle Easterners wore wool often, but linen was especially commonly used by royalty, the wealthy, and the priesthood.
If we look at only one section, we get a limited view.