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.
A metaphor is a figure of speech that uses one thing to represent something else.
In this case, the blackness of the throat is a metaphor for the evil of the hand. Wolves are ruthless predators and scavengers, black is almost always associated with evil. Combining the two images lets us know that this is a bad, bad, person.
The inside of a wolf's throat would be a deadly place to be, combined with the darkness of night you have a metaphor for a violent and evil death. "His hand is as black as the inside of a wolf's throat" renders the hand as menacing and deadly as a wolf's.