Black Techs

is a forbidden technique in which demon contractors be willingly possessed by their contracted demon to draw forth demonic power. It is the opposite of the Zebel Emblem techniques which suppresses demonic power.

The technique was originally developed for humans to fight alongside with demons among the Human World, not for demons birthed from the Demon World especially those on overlord-level. Only Suiten Ikaruga was able to master the technique, unlike spellmasters such as Zenjūrō Saotome due to the technique's incompatibility with his contracted demon, or otherwise destroy the user's body.

In Oga and Beel's case, it was possible for them to master the technique through according to Suiten's hypothesis, due to their strong bond or the demon still being an infant.