The lines in the chorus "that darkness is bitter/this darkness is sweet" are a reference to a Japanese folk song called "Hotaru Koi" ("Come, Firefly"). The original song goes, "Come, firefly/Over there the water is bitter/Over here the water is sweet." Also not to come across as rude, but the translation of this song that appeared in the fan-subbed version of the Shiki anime was wildly inaccurate, so please put it out of your minds.