PVC conduit generally must be buried to a depth of 18".
You can use a box rated for embedment in concrete (typically the watertight outside boxes), but you'll need a deep one to get the thread far enough in to get it past the outer edge of the block. Make up the conduit and box and then pour in your concrete. It may be easier to use a sweep 90 on the conduit and come into the back of an embedded box (instead of below), or use the same sweep coming out of the block enough that you can surface mount the box later. I'd dry fit the threaded end onto the conduit before you fill the block wall with concrete so you don't have to chip it away later.