Blast allows Sensoji to shoot continuous blasts of energy from the palms of his hands at a very rapid rate. These blasts are powerful, with a ton of stopping power, so it is considered an ideal Quirk to shut down most villains.
Although powerful, Blast generates flashes that are bright enough to cause the user to be temporarily blinded by his own power, causing him to have problems with accurate aiming, making the shots unpredictable in range and may.
To counteract this inconvenience, Sensoji only needs to wear shades to protect his eyes, thus preventing Blast from blinding him, giving him a major accuracy boost.
- My Hero Academia: Vigilantes Manga: Chapter 62.