Chrona's weapon attack starts by emitting a high-frequency noise by screaming. This could be inspired by the Excalibur legends, since it was also referred to as 'the singing sword'
One of the doctor's more prominent themes is the screw in his head, which he can be seen tightening - this is likely a play on the expression 'a screw loose' when referring to someone who's not of sound mind.
Blair wore her hat in cat form in this episode, however she did not do so in the manga.
The number to call the shinigami is 42-42-564. In Japanese, that can be read as shini-shini-koroshi, which directly translated means death-death-kill.