I've never seen a concrete block bench for a small mill or lathe anywhere, ever, industry or in home shops.
As wood is so much less effort to work with, and has high strength for a given section/thickness, why use concrete blocks, lightweight or otherwise?
I think they would be bulky inside a shop, and would not take any side loads or vibration as well as a wood or welded steel bench. Both the wood and steel would be more compact structures, leaving more room for storage underneath, and shedding no abrasive dust over time onto stored items like concrete would.
spruce/fir 2 x 4 / 4×4 and a laminate countertop or a 3/4" plywood top would get my vote as easiest and cheapest, with an angle iron welded frame and laminate countertop a close second. Built several of both types and very satisfied with all.
One time I had reason to support the front end of a medium size north American car on a 4×6 foot bench built of spruce 4×4 legs and 2×4 crossmembers screwed together. Car weighed about 2400 lb. The bench supported the front end with no trouble while the suspension was removed and the frame got rust repairs by pathing and welding. Probably more than half the car weight was on that bench. We did it that way rather than on normal jackstands to make it easier for one of the crew who was in a wheelchair to access the work area. The repairs turned out great.
Good luck with your bench. JD