how did blackbeard steal whitebeards devil fruit
In the One Piece series, it is not specifically shown how Blackbeard stole Whitebeard's devil fruit power, but it is known that Blackbeard has the power to absorb the abilities of other Devil Fruit users after they die.
After Whitebeard's death in the Marineford War, Blackbeard arrived on the scene and stole Whitebeard's devil fruit power using his own unique ability. Blackbeard has the Yami Yami no Mi devil fruit, which grants him the power of darkness. This darkness allows him to nullify and absorb devil fruit powers when he touches the person with his darkness.
It is believed that Blackbeard prepared for this situation by having a weapon capable of killing Whitebeard, which caused the devil fruit to reappear and then he absorbed it. Through this method, Blackbeard acquired the Gura Gura no Mi, also known as the Tremor-Tremor Fruit, which grants the user the power to create earthquakes and manipulate vibrations. This made Blackbeard one of the most fearsome and powerful characters in the One Piece series.