Blackwhip grants the user the ability to produce energy tendrils from any part of their body and command them at will. These tendrils are black in coloration with a glowing light teal outline. This Quirk appears to be powered by emotions, primarily anger, meaning the greater the anger, the stronger the Quirk becomes. This was demonstrated during the Paranormal Liberation War, when Izuku used the anger and hatred he felt toward Tomura Shigaraki to increase Blackwhip's power, allowing him to restrain Tomura while he was unsuccessful before.
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.
As Daigoro described it, if not fully in control of their emotions, the user runs the risk of losing control of Blackwhip. This is seen both during and after the War, as the more emotionally unstable Izuku gets, the more out of control Blackwhip becomes, to the point of even enveloping his entire body.
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. While conversing with All Might, he stated that at the time, he was only able to use it for a second, which, while preventing him from fighting like Shota Aizawa or Hanta Sero, was good enough for support techniques.
By the time of the Paranormal Liberation War Arc, Izuku showed greater mastery of Blackwhip, using it to grapple, slingshot and swing off buildings, increasing his mobility and using it to carry large objects and vehicles like buses. Izuku also uses it to attack and grab hold of villains like Tomura, but the latter manages to escape and counter Izuku's attack.
While using a high-percentage smash on Tomura, Izuku also wrapped Blackwhip tendrils around his forearm, acting as a recoil buffer to reinforce his arm from the inside out to keep it from shattering and lessening the damage from using his full power. When his limbs are incapacitated, Izuku shows even greater versatility by launching Blackwhip from his mouth, calling it "Froppy Style" in homage to Tsuyu Asui's own tongue ability.
Izuku's mastery over Blackwhip drastically improves again during his search for All For One and the League of Villains. He can combine One For All at 45%, Blackwhip and Fa Jin to move at speeds comparable to One For All at 100%, with no recoil that would come with it. Later on during these events, Izuku's trained parallel processing allow him to continously sprout up to six tendrils from his back, which subconsciously act as an extra limb for Izuku, needing no more guidance from his arms to maneuver him around.