Are you sure they are cinder blocks and not the concrete type,
Tankslave? They do look almost identical (unless you can get right up on it, before life is growing on it, or lift it for weight) and most people use the term "cinder block" for all blocks without realizing that there is even a difference. I thought that should be addressed to avoid possible confusion.
Moisture causes deterioration of cinder block - this is a known fact in the construction industry, so in building it is used primarily for interior rather than exterior walls. I would imagine that this would be an even worse problem for the more corrosive saltwater. Certainly it takes a while, but I don't know how long it would be before your base started to disappear.
Coal cinders are also known for their outstanding ability to absorb (and slowly release) organics, another reason I don't think cinder blocks would be a good choice for a reef aquarium - you really don't want your base absorbing organics, and later releasing these back into the system.
And I'm not positive how coal cinders and activated carbon differ, but carbon, after about a week of use in a saltwater tank, starts to strip things like trace minerals, so this might be another issue.
All in all, it seems to me that cinder blocks would not be the block of choice in the reef aquarium. I could be wrong - sometimes I am on stuff like this, but my gut tells me this is a problem waiting to happen.
Unless I see something from a public aquarium or research facility, stating that they are using real cinder block and it seems that they do realize the difference (and aren't just using the term cinder because they think it is the right name for what they are using) and do in fact have cinder block, I would be leery of using it. I of course can't tell anyone what to do, but I would avoid cinder blocks and use concrete blocks instead.
