@Domingo:
Uh, I think I phrased it incorrectly, so never mind. I take it that Gray flatlined first and then Juvia performed her magical blood tranfusion using her own… after losing a lot of blood herself. Gray is revived, but blood donor doesn't work that way. Not to mention, it was an asspullish sekrit tech developed offscreen with next to no hint. This isn't The Last Airbender where waterbender can also learn how to bloodbend by training.
In this case there is some reasonable explanation as to how she could transfuse him. She can turn her whole body into water and back. she can also control that water. She could theoretically turn a large amount of her blood into water, put it into him and then change it back. That said… and here is my expectation for revival. It could be said that water can be used to resupply her blood if in fact they are indistinguishable thanks to her type of magic. They are surrounded by ice which is hey water that is frozen. I remember her being able to turn her water to steam, so perhaps she can convert ice to water refill herself and go on with her life. If that is the case, so long as she is near water, she should be almost invincible. Of course all of this is dependent on the writer wanting her to live. If he does. I give it 3 chapters. 1 for the now mad Gray to overpower Invel. 1 for Invel to step it up and fight back, and then have the final clash. 1 for Gray to come out on top and turn to have a touching farewell to Juvia only to find her alive and healing herself with water.