The blackness to me feels like the sacrifice or the consquence of his wish to become a vampire.
He is now in the darkness - without light, without time or context. He has sacrificed all that would give him enjoyment or leisure, all that would remind him of his humanity. He is in a transition from human to vampire and wont take shape either way till he makes peace with his humanity and vampirism, of what he keeps and has to sacrifice. My mind comes up with Louis from Anne Rice's vampire novels as an example. In a dracula movie, Dracula becomes the bat-man when torn between his love for Mina and his own immortality and vengence against God, and neither can work side by side.
Or I could just be waffling as usual hehe.