Plagued proto-drakes are proto-dragons plagued by the new Blight of the Scourge. Although it's unknown from which dragonflight the first generation of plagued proto-dragons were changed from, the following generations were raised and hatched from small blight pools within Scourge loyal frost vrykul villages. The first frost vrykul village to raise these new proto-dragons was Valkyrion, followed by Ymirheim.
Small in numbers these creatures are mostly used as mounts for the frost vrykul that serve the Lich King.
While in the past the Scourge tried to use the black dragonflight for their experiments, these new versions were most likely created from the blue proto-dragons that inhabit the area. Though it's possible the black proto-dragons were used, which would explain their lack of appearance in Northrend among the others that do appear.