At the end of Chapter 214 Deku was able to consciously manifest Blackwhip and use it to save himself from falling pipes. The next chapter hower showed that it fizzled out after a second, sending waves of pain through Izuku's body. Even though Izuku has Blackwhip he still can't use it safely. He could hurt himself or somebody else. This vindicated my faith in Horikoshi as a writer, and my confidence that Deku wasn't suddenly going to be OP.
This hasn't been confirmed yet, but I don't think right now Izuku can use Blackwhip at the same time as the strength/speed enhancement of One for All.
Deku says in chapter 215 "This power...is waiting for me down the road. Once I've got total mastery over One For All....only then can I really start using it." I actually disagree with Izuku about long this is going to take and think he's approaching it from the wrong angle.
This is chapter 216 and Izuku's Full Cowling is at 20%. Izuku has been making slow and steady progress towards being able to use the quirk at 100%. It would feel a little weird if Horikoshi introduced Blackwhip but then after the Joint Training Arc was over it was several years before Izuku was able to use it. It also feel cheap if Horikoshi sped up Izuku's growth so that we could get to Blackwhip sooner.
I think what's going on is a problem similar to what happened when Izuku first inherited One for All. He's wasn't controlling the percent of OfA's power he was putting into Blackwhip. He was either putting 100% of OFA into Blackwhip or he was putting 20% because that's the limit he got comfortable with, while Full Cowling. In either case, the solution is the same. He needs to put a smaller amount of One for All's energy into Blackwhip then build a tolerance.
Deku's 5 yet to be revealed quirks
I think that Blackwhip belonged to the second user of One for All and that Izuku will unlock their quirks in chronological order, culminating in him gaining the quirk of Nana Shimura.