I think the idea of cinder blocks is bad, mortar or not. They will never be flat or fit perfectly. A purpose- built stand for turntables is a better idea. Look at Adona racks. It will look and perform better than blocks and you'll be happier in the long run. Cinder blocks are for college kids.
concrete (cinder) block tower with wood chopping block for turntable stand
Any suggestions from users who have done, or contemplate doing this. I looked at concrete blocks at Lowes last night. I made two stacks of 3 side by side, plus a solid block (without the center openings) as the top block. This gave me about 34" in height. Then the thought is either a Walnut or Bamboo chopping board for a top shelf. I would use four cork or cork/rubber sandwich (2"X 2") squares to couple the wood chopping block to the concrete tower. I am also considering an Isoacoustics platform as a more expensive alternative. Fire away....?