The Black Thing is the power of evil, even Evil itself. L'Engle presents darkness as the absence of light, rather than just nothingness. Dark is totally opposite of light. "The light shines in the darkness, but the darkness has not understood it." (John 1:5)
Earth is presented as a "shadowed" planet, in which the darkness surrounds it, but has not gained total control over it. This is the cosmic battle of light versus darkness, good vesus evil. Mrs. Whatsit is presented as a star that has given up itself in the fight against the dark. Yet she has not been totally destroyed, because light cannot destroy dark.
The ultimate manifestation of the dark is hate. It is through love that Meg is able to rescue Charles Wallace from the dark power of IT. "Anyone who claims to be in the light but hates his brother is still in the darkness. Whoever loves his brother lives in the light, and there is nothing in him to make him stumble. But whoever hates his brother is in the darkness and walks around in the darkness." (I John 2:9-11)
The planet Camazotz is a planet that has totally submitted to the dark. There, darkness is manifested by loss of freedom and individuality. What the people are is shrouded, as if in darkness. Shining the light on each person as an individual leads to freedom.