Jim Cleary : Plastic sheeting will likely not hold up under these conditions due to its poor high-temperature tolerance. For example, HDPE plastic has a working temp rating of only 160F and Polypropylene is 200F. While your water tank may not attain 200F temps, the cinder blocks will be over 1000F so the plastic will break down on the hot side. Additionally, cinder blocks are very susceptible to thermal cracking, so when the plastic springs a leak, the cinder blocks will crack due to the rapid temperature change and lose their structural integrity, which may result in a catastrophic tidal wave in your basement.
As an alternative, can you use the same storage tank, but cycle water into it away from the heat source? Is it possible to extract enough heat energy from the flue by coiling 3/8" copper tubing around it in conjunction with a very small 1/12 HP pump (approx 7gpm)? This would be an open system (both ends of the copper tubing terminate in the holding tank) so there is no issue with pressure. Calcium deposits in the copper would be minimal since the same water is continuously cycled. Evaporated water could be replaced with rainwater.
Water is hard to beat as a thermal battery, so I would like to see a working solution.