It's an interesting question, especially since some common concepts of bedouin have them in black robes.
The reason is that suprisingly, baggy dark clothing is far better than baggy white in a environment with so much as any wind above 7 miles per hour (or 3 meters per second).
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Under cold conditions with no wind speed, black, flattened plumage held in heat the best (though barely, compared to fluffed black plumage). Under hot conditions with no wind, white, fluffed plumage let heat escape the best. Both pretty logical findings.
But once the wind picked up, the results changed dramatically. With even a modest wind (anything above 3 m/s, or about 7 m.p.h.) fluffed white plumage exhibit the lowest net heat loss. This explains the large number of arctic animals that are fluffy and white. It's not just camouflage. It also explains why we do not see white animals with fluffy fur in the desert, if white was truly the best at heat-decreasing.
At high temperatures, as I say, white is best at not transmitting solar/ambient heat to the skin when windspeed is zero (only barely better when fluffed). However, with an increase in windspeed (again anything above 3 m/s), fluffed black plumage is the best at reducing the amount of heat transmitted to the skin. Flattened black plumage is the worst in terms of heat gain no matter what the windspeed." From the Straightdope website -
http://www.straightdope.com/mailbag/mblackcool.html Is a link explaining it better than I could.