mounting 75lb plasma tv

No studs at all? What's the plywood secured to? That 1.25" gap between the plywood and concrete block makes it sound like there are some 2x4 or 1x3's attached horizontally. Can you get a wire coat-hanger into your small hole and probe around with that?

If you want to do it very securely, I would recommend cutting out a square of drywall and plywood behind the TV area, attaching wood to the concrete blocks with tapcons, and then putting the cut-out pieces back in place and attaching the TV mount to that. Or you could replace the area you cut out with plywood and then paint that the same color as the wall.

On the other hand, 1/4" is pretty thin for plywood but if you are using multiple bolts, the wall is firm otherwise, and you are only using a tilt mount (not a cantilever mount where the TV is on a swing arm), I don't think you'll have an issue.