I would use dirt as my backer. It would try to keep the 'curve' in the face of the blade. I'd think concrete might flatten it out more face-wise.
It's just a poor man's fix anyway. That blade will never be the same strength and it would take a day of pounding just to get it back looking right. You'd almost be better off starting with some cut flat stock and have someone with a roller set roll the face curve into it and mount a new cutting edge.
I'd personally pitch the blade in the scrap/reuse pile and use the mount for something different like a trencher or something else I could fab up.
