The Saudi Arabian are basically itinerant and they have been nomadic in nature since centuries. These people have profitable businesses and earn a lot from trading between Africans and Indians or Persians. The percentage of profit was really high and anybody who can loot them can become rich by stealing their precious possessions.
Few centuries ago, a Saudi caravan was carrying African weaved textile with them in its entourage and they were attacked by the robbers. The person responsible for the caravan’s security immediately informed the warlord about the attack and he ordered all the women to drape themselves in those black clothes so that bandits could not spot them easily, the main reason was obviously to avert any tragedy like rape or abduction.
Later the story goes like that most of the women accepted black robe as a move towards personal security. But now it has become a social symbol for Muslim women.
After few years, it was officially announced by a group of warlords that women should wear black robes while men must wear white clothes. So, black robes worn by women shows that it is basically a step towards safety and nothing else. But this practice is prevalent in Islam and mostly burqa-clad women are usually Muslim.