It is my moral duty to warn you:
1. NOT a good idea to get underneath the vehicle when the weight is supported only by a jack, without stands.
Jacks can fail and drop the vehicle without warning.
2. I wouldn't recommend using cinder blocks to support the vehicle, either: I have seen cinder blocks fail without warning also, under compressive load without proper support around the sides of the block.
Concrete generally has high compressive strength, but cinder blocks are a special case - they need unyielding (rigid) support around all sides, and broad load area distribution, to support the heavier loads.
3. I've also have seen 4x4 PT lumber fail under compressive load and drop the vehicle, when used as spacer to jack vehicle.
FWIW.