I have one very similar, albeit with a much smaller TV (32"). You wont be able to attach it to the plasterboard, its too big and heavy. You could go through the plasterboard into the block and keep it flush against the plasterboard but this could possible crush the plasterboard with the force pushing against it.
I cut out a square of plasterbaord and then attached directly to the block wall (internal breeze). However, this means that it wont go flush against the wall then folded, because the bracket now presses against the edge of the plasterboard. You would have to cut a much larger hole to allow the bracket to go flush and that would leave a bit of a mess. You would then need to try and plaster the block and adding in some edging. Think that will be a right fiddle.
Probably your best option would be to cut out a length of plasterboard between two studs. Add some noggins where you want the TV to go (possible with some diagonal support braces also). Then put new plasterboard over the hole, tape, and plaster the joint and paint. Then fix the mount directly to the noggins.That way the mount if flush to the plasterboard, but has support directly behind it to take the weight and pressure.
ETA - or mount directly on the noggins, and then plasterboard around it so its flush with the plasterbaord, rather than sitting on top.