Toogle bolts are a good bet. There is this epoxy type plug that works well. What type of block are we dealing with. Is it the red clay block. I would try to hit a mortar joint and place those plastic plugs with some screws as well. If it were my house i would cut back the drywall and mount a two by twelve to the wall first, I would also power grab it to the wall on top of all the toggles and rawls or what not. You will not see it behind the t v. It would also give you a place to mount an outlet and a strip to hold all the hdmi cables and what not.