The black oil sunflower seeds are just a variety that is used to make sunflower oil and is also a popular bird seed for wild birds. It is very rich in nutritious oils that are good for putting on weight, and making coats glossy. Just a little goes a long way -- think of it as a supplement. They also do have a good amount of protein. Again, think of it as a supplement.
Goats can eat any type of sunflower seeds -- the black oil type just has more oil in it.