Concrete blocks are stupidly strong, even if placed the "wrong" way, i.e. with the open voids visible from the sides. I always use them that way and utilize the voids as handy little storage cubbies.
I'd be more concerned with making sure that the floor they stand on is not creating any uneven support, and I would put a sheet of plywood on top of the blocks. This isn't so much for support as it is to make it easier to see if you have a level surface. If the plywood is flat to start with, and if you find it wobbles corner to corner, that "twist" applied to the bottom of the tank is the kiss of death. A little bit of lean to one side, or back to front, is unsightly but not really dangerous; but that twisting force is bad. Check the mating surface of each block for bumps and casting blemishes, which can create these stresses but which can be easily chipped or knocked off to prevent the problem.
I don't bother much with concrete blocks anymore; using 2x4 and 2x6 lumber it is easy to construct a stand that is sufficiently strong, easy to level, and much less wasteful of undertank space. To my mind, the only real benefit of the blocks is that you can literally create a stand in the time that it takes to cut one piece of plywood to size and then stack the blocks up.
