I'm thinking it would be great for support. In my head I have a few issues with cinder right on top of the carpet though. One would be moisture and mildew/mold. No matter what we do, sometimes we drip water from our tanks and cinders can hold a huge amount of moisture. Second I can see terrible staining of your carpet underneath. Any substance/dust on those blocks and add any moisture it will wick to your rug.
Just a suggestion - Home Depot sells white vinyl garage trim. It is 8" x 8 or 10 feet. They have two thicknesses, 3/8" or 1/2" thick, your preference. The face side has a wood grain to it. It is sold as garage door trim, but I use it for a million things because it is easy to cut/rip. If it were me wanting to do this, I would make a vinyl 'perimeter pad' to set the blocks on. Pretty cheap too, IIRC $12 a length.