This might be an appropriate time to address another of those 'myths', 'Old wives tales' associated with ponds. That anything containing limestone will adversely affect the pH of a pond's water column.
Limestone is primarily Calcium carbonate. Sound familiar? It should as it is the mineral that KH is derived from. Adding limestone, in any form, to a pond will increase the KH which, as we all know, serves to stabilize the pH. In and of itself,KH will not raise or lower the pH of a pond's water column.
One ounce of lime will raise the Alkalinity approximately one (1) point for every 4500 gallons.It is used qu9ite often to stabilize the pH in fishery and farm ponds.
Any leachate occurring from concrete, either poured or block form will be well below this above given rate so it would be slow to even affect the Alkalinity (KH).
Additionally, lime (Calcium carbonate) will add a certain level of CO2 to the water which aids in photosynthesis.
One could infer that concrete blocks could be a beneficial addition to the overall water chemistry