Thank you for your question. Here is what one Extension source says:

"Cement block, cinder block and concrete block, all are made with cement and fine aggregates such as sand or small stones. Fly ash is also often included. Fly ash is a byproduct of burning coal and so contains heavy metals and other hazardous waste. Labels do not give specific information on exactly what aggregate is used in the manufacture of the block. There is also little research data on this topic. Ultimately, this becomes a personal choice based on your comfort level. If you plan to use block as a raised bed material -- and many people do – and you are concerned about potential risks, you could seal the blocks with polymer paint. Or you can choose to use another material."

https://extension.umd.edu/hgic/topics/materials-building-raised-beds

Hope this is helpful.  Good luck!