Blackwhip grants the user the ability to produce black energy tendrils from their arm and command them at will.
Daigoro describes Blackwhip as a top-grade Quirk. It appears to be useful for capturing enemies and increasing one's own mobility by grabbing onto objects.
While Daigoro was alive, he used Blackwhip primarily for mid-air maneuverability and enemy binding.
By the time Izuku acquires this ability, Blackwhip has evolved within the core of One For All, drastically increasing its potential power and scope. When Izuku focuses on capturing a target and gets angry, this ability explodes out of his arm. The black tendrils are much larger and powerful enough to cause Izuku great pain and force him into submission. 
Later, when Izuku used the Quirk again to grab onto some pipes that were going to hit him, he found he could only use the Quirk for a brief amount of time because of how powerful it had become due to One For All. He was left with a numbing pain after using it.
After the Joint Training Battle, Izuku gained the ability to use a less powerful and smaller version of Blackwhip without any physical strain, as long as he stays calm while using it and doesn't go over five percent of his power. By the wane of the Hero Work-Studies: Round 2, in Izuku's fight with Ending, he was able to use Blackwhip to save cars from falling. At the moment, he can only use it for a second, which prevents him from fighting like Shota Aizawa or Hanta Sero, but feels he can use it for support techniques.
- ↑ 1.0 1.1 My Hero Academia Manga: Chapter 257.
- ↑ My Hero Academia Manga: Chapter 213.
- ↑ My Hero Academia Manga: Chapter 211.
- ↑ My Hero Academia Manga: Chapter 215.
- ↑ My Hero Academia Manga: Chapter 241.
- ↑ My Hero Academia Manga: Chapter 251.