The black Reimu is described as the second side of Reimu. Because Reimu pushed her lust into her unconscious, the black Reimu was created as a second personality within her unconscious. It says that even though people are pretty much supposed to have two sides (that yin yang orb page), the lust she was hiding was too big so her mind wasn't balanced. It seems that Yukari felt that that state was dangerous so she split off the black Reimu from Reimu and granted her a body.