Blight drones are produced by Forge World with rules published separately. They are supposed to be created in the image of, and possessed by Nurgle. Significantly, they're about the only viable fast attack choice for an Epidemius Army and add some much needed firepower to it.
But here is the twist. Codex: chaos daemons states that the Tally of Pestilence that fuels an Epidemius Army only works for models "killed by followers of Nurgle (i.e. any daemon of Nurgle, or model with the Mark of Nurgle)". Nowhere in the rules for the Blight Drone does it state that is has the Mark of Nurgle, or is explicitly a Nurgle Daemon. So: do blight drone kills count toward Epidemius' tally?
On the other hand, nowhere in codex: chaos daemons do they explicitly define what models belong to "daemons of Nurgle". The blight drone could well fall under that category given it is (clearly) possessed by a Nurgle entity. Comments welcome!