Years ago, Blackbeard read about the Yami Yami no Mi, and hunted for it. He joined the Whitebeard Pirates as he believed his chances of encountering the fruit is best while there.[1]
One day, Thatch, commander of the Whitebeard Pirates' fourth division, found a Devil Fruit. Recognizing it as the Yami Yami no Mi, the very reason he joined the Whitebeard Pirates for, Blackbeard killed Thatch to get the fruit and fled.[1]