In the Wordless book, white is about purity and black (correctly darkness) is about sin in a figurative sense and not referring colors on a pallet or people’s skin. The pigmentation of a person’s skin tells us nothing about what is in the heart, the inner man.
We should think of white page as a description of purity like in Psalm 51.7 when David asks God to,
“Purge me with hyssop, and I shall be clean;
wash me, and I shall be whiter than snow.”
The dark page in the context of the wordless book seems to be in the sense of uncleanness or the dirty stain of sin (also see Isaiah 1:18 where sin is described as a scarlet stain).
Darkness is a scriptural description of the realm of sin and Satan (Col.1:13). In John 3:20-21 describes those who do evil as loving darkness because it hides their deeds. In that context, it is the opposite of light. Darkness is used in Ephesians 5:8 & 11 to refer to the works that the sons of disobedience do.