I finished the Blood and Ice murder quest and STILL cannot get Shahvee to offer her favor quest.
I did notice one thing - based on one of the wiki's she is part of some 402FavorQuestGiver faction, and I noticed that the only OTHER npc in that faction was, in fact, someone I was doing a favor for.. It was the one for.. his name starts with an A, avanor or some such, and he wanted a sword or something of his family.. He's in Whiterun. Perhaps there's some sort of 'cool down' for being offered the favor from that same set? I completed his task, but she didn't offer me a favor, even still. Maybe some folks never took his favor, and thus, got hers? 100% shot in the dark, but hey.
I likely will be starting another character soon and I will avoid his favor task just to be safe. Sadly, though, Shahvee continues to elude my affections.