There are several reasons why the Abaya is black:
Women utilized black goatskin to make clothing in ancient times when resources were scarce. Because goatskin was readily available, ladies made abayas in a black hue.
The black Abaya does not highlight the body in direct sunlight.
Black is an unappealing color. As a result, all Islamic apparel, such as the burka, hijab, and Abaya, is black.