@MickeyFrogeater My gut is telling me "no" on this one mostly because fel energy is entropic. :)
Judging from this Tweet from Loreology: Fel is Entropic yet Plague is not.
So it seems Fel is Entropic/Death energy while Plague seems to be Life energy(possibly mixed with Nature since Blood Magic is said to be pure Life magic).
Of course it's obvious that both Life and Death spells do Shadow damage which is either because of Game Mechanics(most likely) or because both Life, Reality and Shadow come from a clash of Light(which does Holy damage) and Void(which like Shadow, Fel/Entropic Energies, Plague and Life does Shadow damage).