In chapter 213 we finally learned the answer to what happens when you gvie One for All to somebody who already has a quirk. It turns out that when you give One for All to a person whow as born with a quirk that it combines with that preexisting quirk. This makes sense when you think about One for All's origins, it was created from the merging of two quirks; one that "stockpiles power" and one that allows it to be transferred.
So can Deku use Black Whip now?[]
I've heard some people claim the revelation in chapter 213 breaks the underdog narrative or ruins the story but I disagree, It's already been established that Izuku having a quirk is something very different from being able to use it safely and effectively. Remember the first time Izuku used One for All? He destroyed the robot but broke three of his limbs in the process.
We saw that Deku had no control over Black Whip and I don't think his visit with the goggles guy has fixed that problem. I know some people are hoping Izuku will be able to use Black Whip against Shinso in the upcoming fight, but I don't think that's going to happen. I think all that's changed thanks to his chat with goggles guy is that now Deku understands how the quirk was activated. Deku first activated his quirk during the Entrance Exam but it took until the Stain Arc for him to be able to fight with it without mutilating himself.
Based on what we know of Deku's personalty he might spend some time shunning this new power. Unlike his super strength, this ability doens't seem harmful to Izuku himslef but his lack of control over it is hurtful to any friends or bystanders near Deku when he unleashes Black Whip. I don't think it'll take Izuku quite as long as to be able to use Black Whip but it would feel incongrous for him to be able to use it right away.
How will this effect the story?[]
We already know that by the end of the story Deku will become the world's greatest superhero. Until now Deku's progression in terms of power was just him learning new ways to use his augmented strength/speed and raising the percentage of One for All he was capable of using.