The Flash Season 2: 28 Easter Eggs & References You Might've Missed
26. Blackgate
Sand Demon was a pretty forgettable villain early on in Season 2 (more like BLAND Demon, eh guys?!), but he did at least provide a nice little reference to the wider DC universe.
When Joe and Patty arrest Eddie Slick, the Earth-1 version (believing him to be the Earth-2 metahuman one), they question him about the night the particle accelerator exploded, and he reveals he was in Blackgate.
While Iron Heights is the prison we're used to seeing on The Flash, Blackgate Penitentiary usually belongs to Batman, serving as Gotham's alternative to Arkham Asylum.