A linguistic discrepancy in action. The words of two very different shades come from the same origin. Iirc, black (en), blanc (fr), blanco (sp), bianca (it), biały (po), all come from the same roots, that is, a flash. Some describe the flash itself, while some describe the powdery residue left behind.
The thought of bleach being white might be what is thought of as the result of the action of bleaching something.