If you use them you would have to cure them to leach out the ALK from the lime. Concrete blocks are made from cement, lime and sand, gravel and pumice.
If you look into them you will see that there are different ratio's in the way they are made. Some are more porous and lighter than others, labeled as a 60/40 or 50/50 ( I don't remember the exact details, I was a brick layer's hod carrier near 20 years ago) the lighter ones will have more surface area for bacteria to grow as they are less dense and have more pumice.
Having said that, Yes, IMO blocks are reef safe.
But, if the blocks are made with silica sand they could cause diatom problems in the long term. I suppose, if you buy your blocks from a masonry company (not Home depot) like Mutual Materials in Bellevue you could easily trace them back to the manufacturing source and find out exactly what they are made of.
Instead, I would suggest using a PVC pipe structure to hold your rock work up or acrylic rod like already suggested. It will provide your fish with much larger caves to hide in.
If you take a look at page eight of my build thread
here, you can get an idea of my PVC rock rack to hold the rocks up in a 30" deep tank.
