First time since "Resist (2017)" that Kara appears solely as Supergirl, and not as her civilian secret identity.
In the DC comics, the Blight is a nomadic race of techno-organic beings, who assimilate new races in search of immortality, but turning everyone into perpetually decaying techno-zombies in the process.
When she's united with Purity and Pestilence, Reign says "El mayarah". The phrase is Kryptonian for "stronger together", and is what the House of El coat of arms - the "S" Kara wears on her suit - means.
Pestilence's DC comic counterpart may be Perrilus who was a World killer from the Supergirl New 52 comics who also had the same abilities to creates plagues and diseases.