Hey Josh,
Yes, your set-up there is good on stability in the sense that it is not too likely to tip over (flat portion of the frame resting on flat cinder blocks/wood). However, it is the strength of the cinder blocks being of concern. Your car may very well never tip over from being imbalanced, but like others mentioned, the cinder blocks are not strong enough to take the load and if or when they fail, there may be no warning.
You can achieve decent balance with jack stands too, you just need to put some extra focus on where you place the wood going between the top of the jack stand and the frame.
It also never hurts to have redundancy, and if you're after something big and flat and stable, you could put whatever amount of wood you want piled in a similar fashion to your cinder blocks under additional corners of the car/frame next to the jack stands.
In order of the materials in which I would trust the strength would be steel, then wood, then cinder blocks. And the cinder blocks are a distant third.