Black allows the user to merge their body into anything that is dark-colored, whether naturally dark, or dark via external circumstances, such as shadows. They can also move themself through the darkness freely.
This ability does not take mass into account, so merging with small or thin dark objects, such as a mushroom or a cape, is entirely possible.
Objects that are naturally mobile can be forced to move if the user assimilates into them. Because of this, Black holds an advantage against Dark Shadow, a dark-colored living appendage, which he is capable of possessing and then controlling its actions, regardless of Fumikage's wishes.
This Quirk is most efficient in cramped areas with little to no lighting, where shadows are abundant and the lack of space physically hinders the users opponents, preventing them from fighting properly or escaping.
Brightening up a shadowed location will quickly cause the user to be forcibly ejected from any no longer dark object when the shadow shrouding it vanishes. As such, it has a weakness to light-based Quirks.
It also appears that the user can keep the ability "on hold" while looking for something dark to merge with, which an enemy may exploit by forcing their body into a dark object they are able to grasp (which prevents the user from leaving).
Shihai originally could only merge himself with black objects, but after strengthening his Quirk, he is now able to move the incorporated black objects themselves, although this can only be done with objects that were originally capable of mobility.
In combat, Shihai uses Black to hide from enemy attacks or to execute ambushes. Once merged with a black object, Shihai can move through its whole structure at very high speeds, even while carrying a person.
- Black appears to function similarly to Mimicry, although it has some key differences:
- Black is limited to objects which display a certain color. Mimicry does not have this limit.
- Mimicry is limited to objects of a certain size, smaller than a refrigerator (larger objects could only be possessed through the effects of Trigger). Black seemingly does not have this limit.
- With Mimicry, Joi Irinaka could manipulate the object he was currently possessing in any way he wanted, changing even its shape. Shihai had to improve his Quirk first before being capable of doing so, and even then, he's limited to manipulating only mobile objects into moving.
- Since Black and Dark Shadow are darkness-based Quirks, they share the function of being more effective in darkness, as well as the weakness for being less effective when exposed to light.
- A young girl named Yuka manifests a similar acting Quirk, able to meld into the shadows and control objects in the dark, in the first School Briefs novel.
- Black - Wikipedia article about the color this Quirk's abilities are based on.
- My Hero Academia Manga: Chapter 198.