I have been using bricks in my pond for years without a problem. No mortar on them.
Cinder blocks are probably inert. If actually made with cinders from burning of coal, which produces tons of cinders, they can be OK. I know that ponds in WV that served as dumping places for cinders actually had good populations of sunfishes in them, but there were signs saying not to fish around the pond, possibly because the fish were adsorbing the toxic metals that would possibly come from the cinders.
The mortar used to hold the cinders together would do as others here have said, cause the pH to go up. SO it would be hard to use them in a tank you wanted to hold at below pH 7.
If you do use cinder blocks, I would recommend they be placed in water for a week or two prior to use in the aquarium.