if the concrete blocks have not been painted, use a pressure washer to throughly clean the surface that will be tiled. use a thinset mortar with an acrylic additive to seccure the tiles to the wall. the home depot sells the thinset and an latex additive called ad-mix. when used with the thinset, it gives a very strong bond and also helps the tile to adhear to the wall. the block wall would have the thinset applied with a notched trowel the same as is done when tiling a floor. to get best adheasion, also apply a very thin layer of the thinset to the back side of the tiles, using a straight edge of the trowel, remove almost all of the thinset, leaving only a thin film on the tile.
this has worked for me and the tile will normally hold in place with no problem during installation. also, buy a bag of the + spacers to insure that the tile grout line is left open.
good luck on the tiling
