Yes, Blackbeard did eat two Devil fruits. He ate the Darkness frutas (Yami Yami no Mi), and then roubou Whitebeard's Quake frutas (Gura Gura no Mi). Also, considering the fact that he and his crew are hunting Devil fruits, I think that it is seguro to assume that he may be able to eat more.
Most people cannot do this as the two 'devils' inside of them would kill the person, however Oda Sensei has said that Blackbeard has a 'special body' so he can do this. Many have theorized that this is because of the Yami Yami no Mi. Perhaps one ability of this frutas is the ability to absorb everything, even other Devil Fruits.
If this was the case, it would explain why Blackbeard wanted this frutas so badly -- badly enough to steal it from Thatch, kill him and be hunted down por his fleet commander.
