Ralph says he's "not looking to get married" when he says he's going to find Sue Dearbon. In the comics he's married to a woman named Sue (Dearbon) Dibney.
Frost uses the term "Post Crisis," which is a common fandom term for the time after the comics' Crisis on Infinite Earths.
Joe mentions Chyre, his old partner from Pilot (2014). Because Ralph used to be a cop, he was also acquainted with Chyre.
When the hospital's supply of blood is stolen, Joe confesses he's afraid they're dealing with some kind of "meta vampire". By the end of the episode, this is more or less what Ramsey has turned into.
First Halloween episode of The Flash. This episode actually will air two days before Halloween in US markets.