Any idea what I'm drilling into?
Cinderblocks, as you call them, are also known as concrete masonry units (CMU). The “wall” thickness of CMU’s is 1 1/4”, so it’s probably not CMU’s if you think it’s 1/2” - 3/4” thick. 

Could be plaster...it’s about 3/4” - 7/8” thick. Or, it could be thin coat plaster over wallboard. Usually the wallboard is 1/2” thick and very hard (dense) plaster about 1/16” thick is applied directly to the wallboard. 

Plaster, thin coat plaster, wallboard, etc. are not structural and would not be suitable to support your TV.

I’d look for another stud and follow the manufacturer’s recommendation about connecting into studs. (Do they give an alternate installation layout using a piece of plywood that spans from two studs?)