This is a unique ability of Gyūki that allows it to produce ink. Though Gyūki is the sole inheritor of the ability, it can be passed on to its jinchūriki.

Jinchūriki Usage

• Killer B has made use of the ink by ejecting it from his mouth to blind an opponent or to create ink clones that can be used to seal targets. Killer B has also made a more practical use of the ink by placing some of it on his finger to use as a makeshift pencil.
• Naruto Uzumaki has made use of the ink by mixing it into his Wind Release: Rasenshuriken while in Six Paths Sage Mode. Whether the ink is ejected from his mouth or formed from thin air is unknown.