The term bleached can have different meanings depending on the context. In general, it refers to the process of removing color from a substance, typically through the use of chemicals or bleach.
In the context you provided, bleached is used metaphorically to describe a black woman who has had sexual relationships with white men. In this sense, bleached implies a change or transformation in the womans identity or behavior as a result of her relationships with white men. This usage is considered derogatory and offensive.