More than a simply building, The Black House is indeed a sentient living creature linked in some way to The Devils, serving in many ways to the affairs of those creatures.
The House is part of the ancient ritual of the Blue Night festival, known by Magic Users as the place where a Devil oversees the partnership contracts signs between sorcerers, huge in the inside, seemly able to warp reality inside it and even having its own pocket dimension, many rooms and the ability to change locations or teleport the people inside of it.
Not much is known about how the entity interacts to others, it is known only Devils are able to communicate with it through its core inside the building, doing and answering what people want from it. Seemly polite in the way it respond too and even going so far that without the need to ask it serves tea to its guests.
Chidaruma, as well many other Devils came from the House to En's mansion in order to meet and give their cards to the newly discovered Magic User able to control time, Kaiman and Fukuyama where at Kesber Park at the moment, greeting the Devils from far away.
Nikaido was forced to sign a contract with En inside of the main room of the House, as Asu helpessly saw how her chest was open by the Devil lord, Shin and Noi signed their contracts there too, Fujita and Ebisu could not as the little sorceress had stomachache and spent the rest of the night inside the bathroom.