Board Thread:Warhammer 40k General Discussion/@comment-6134883-20130417202949/@comment-8257990-20130419071220

Kadjah, no blood magic was involved whaotoever it was merely the fact that the blood of the sisters was so pure that it helped to face the corruption.

As for your second point that was my bad I could've elaborated better, the corruption came in the form of a tide of almost black blood that came from Khornes realm directly and even the Grey Knights were cautious as the knew that the may have lost themselves to Chaos upon contact with the blood, and they were the only ones that knew how to stop the "bloodtide." They realised that the sisters or themselves would've become corrupted if no action was taken and so they decided that if only they could stop the blood tide then they could kill the Sisters to remove any risk of corruption...