This comparison between the blackness of a hand and the inside of a wolf's throat is a simile that also contains synedoche. And, as mentioned, the blackness of the hand is a metaphor, as well.
Here is the definition of simile: A figure of speech that makes an explicit comparison between two unlike things, using a word such as like, as, than, or resembles.
The comparison between the hand and the wolf's throat is explicit and employs the word as. The synedoche, a figure of speech in which a part represents the whole, is hand as it represents the entire person who is evil.