To start off so I don't waste other classes times, this is a trick that works for rogues, and possibly for mages, it only saves you taking more damage than necessary.
When I was doing this quest for the daily, I found something out that might already be common knowledge, and is probably already working as intended, but I figure I'd share a little trick about this quest.
For those who don't know about this daily quest, in 2.4, Blood for Blood is located in the Throne of Kil'Jaeden in the Hellfire Peninsula. This quest has you collect Demonic Blood from the Wrath Herald's nearby, then use it on the Felblood Initiates by using the fel siphon to weaken them and kill them.
Where the trick comes in, is by sapping a Felblood Initiate, then using the Demonic Blood charge on it to weaken it. Once the fel siphon channel is finished, you then attack, breaking the sap, and they'll continued being stunned as the effect of the blood produces, and turn into the Emaciated Felblood all without having laid one hit. As mentioned before, this works as intended, but it's just a way for the rogues to avoid a little bit of damage, preventing them beating on you while you channel the blood.
This part I have not tested, due to not having a mage, but if Sap and the blood produces the effect I described, I assume that sheeping and applying the blood will yield the same effect.