Blue Sky Sauce is a bright blue, possibly poisonous sauce that Kuroyuri often uses on his/her food. When normal humans ingest the sauce, they will faint or at least feel nauseous, due to the 'amount of flavour' which is present in the sauce, but Warsfeil seem to be able to take it without suffering any ill effects. The sauce is stored in small bottles. It made its first appearance in the special chapter Platonic Cafe.
In the 99th and last Kapitel, Kuroyuri plans to sell the Blue Sky Sauce, though it's not shown whether he/she succeeds and Haruse expresses uncertainty about the viability of this plan.