The ability of dark fruit has two functions: first, it can make other demon fruit people unable to use this ability, that is, temporarily lose this ability (within the control of Blackbeard), and second, it can absorb the ability of dead demon fruit (after Blackbeard died, Blackbeard absorbed the vibration ability and became the first person with two abilities in the era of the Great Pirate). Ace was defeated because he didn't know how unknown Blackbeard was. Luffy only fought Blackbeard once in Tucheng, but both sides fought once, and then stopped there because he wanted to save Ace. In addition, it is not a black beard that can absorb all the abilities of the dead. White beard absorbed the ability of white beard after his death and vomited a lot of blood, which proved that his ability to absorb a demon fruit did great harm to himself! So the ability to absorb white beard is already the limit! Because although darkness is terrible and can absorb everything, he has to suffer twice as much as ordinary people. If the pain is too great, he will die!