Yes, Blackbeard did eat two Devil fruits. He ate the Darkness ফলমূল (Yami Yami no Mi), and then গাউন Whitebeard's Quake ফলমূল (Gura Gura no Mi). Also, considering the fact that he and his crew are hunting Devil fruits, I think that it is নিরাপদ 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 ব্যক্ত 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 ফলমূল is the ability to absorb everything, even other Devil Fruits.
If this was the case, it would explain why Blackbeard wanted this ফলমূল so badly -- badly enough to steal it from Thatch, kill him and be hunted down দ্বারা his fleet commander.