Unless stated somewhere, I don't really believe that the dark seducers are really the same daedra as Nocturnal's seducers from Battlespire. One reason being, the dark seducers are really known as Mazken, while Nocturnal's seducers are not (correct me if I am wrong). Of course there is skin color difference, but that really means anything.
I always assumed the Mazken from SI were the same as Dark Seducers from Daggerfall and Battlespire. Different in many ways, sure, but because they skipped MW I just figured that little things like how they were represented in previous games don't really matter. Look at the Dark Elf Queen Ben-Hur. In DF, she is clearly not a Dark Elf. In Tribunal (unless I'm thinking of a mod that adds her?) she is clearly Dunmer. Same person, different game.