Is a black snake poisoning

They may, in that they prey on snakes as well as other reptiles, birds, small mammals, eggs, insects, etc. But they do not specifically prey on poisonous snakes. Typically people like it when they find out there is a king snake living near their house for this reason. They are also nice for rodent control.

There are a large variety of snakes often called black snakes, but may have very different life histories--from pine snakes to the venomous cottonmouth moccasin. However, this addresses a whole different topic.