Is she obsessive about other things? Does her food have to be on the plate just so? Does she have to put on her pants first and then her shirt? If so I recommend a psychological evaluation.
Barring other obsessions, I suggest this has become a habit with her and it can be broken. Stop feeding into her demands. Tell her you know she can figure out how to place her blankie and then leave the room so that you don't get caught up in her drama.
Be sympathetic. Be calm and affectionate. But tell her you're no longer helping her with her blankie and stick with it.
YOu might try holding her and her blankie and talk about how the blankie makes her feel. Good naturedly place the blanket in various ways and say, something to the effect, "this blankie feels good right here." "This blankie is so warm and cuddly here and here and here."
This sounds like her blankie is extra important to her as a source of security. She doesn't feel safe unless its on her just so and she can't make that happen. This seems to indicate she has little faith in her own ability to take care of herself. Perhaps working with her on ways to develop self-confidence might help. Praise her when she does other things. Help her to be more independent while at the same time reassuring her that you're there to help her.
This is a tough one. It feels a bit extreme to me even after reading other's descriptions of similar situations with their child. I suggest it means finding a middle ground of helping her feel your support while encouraging her to handle the blankie on her own.