Ok, thought I would give an update on how my work has went so far.
I finally got the pool and started some real work yesterday after a couple of false starts, a vacation, and several other projects creeping in. My kids are about to throttle me by now
This is pretty hard clay, not real unlevel, maybe 8" max from the high end is all I need to dig off, existing sod. I have a small MF 1010 (~1300lb) tractor, no additional weights, turf tires, 2WD (diff lock), and a 4' rear blade with no extra weights and no TNT.
I can report that I am having pretty good success, although its definitely taking me longer than if I had something with scarifiers or if I had a FEL

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Yesterday in about 3 hours time I was able to scrap off all the remaining sod and excavate about 2.5 - 3 yards of dirt, load it up into my little trailer with my manual FEL (aka shovel) and fill in low spots around the yard.
I still have some leveling to do, but I think in another 3 hours or so I will be to the point where I will be fine tuning.
Things I have found out with this experiment:
1. A rear blade definitely can cut hard clay and remove/move a lot of soil. In fact I find traction to be my biggest limitation and I often have to lift the blade up after its dug in too deep.
2. I messed around with different tilts and found that just slightly more aggressive than pure vertical works great. Too aggressive digs in too deep for me and I sit and spin. Pure vertical doesn't dig in enough.
3. With the proper tilt I turn the blade at an angle and it cuts in at the one point as I turn and will actually flip the dirt almost like a plow. I've seen it dig in and flip dirt up to 2" thick on one pass. Anything deeper than that and I spin.
4. Dry soil is a lot easier than wet soil. I tried wetting the soil a couple of times before I went on vacation and had only marginal success. Yesterday I left it bone dry and had really good success. I think its a couple of things, when the clay is wet it is softer but it sticks more, but when dry it crumbles and breaks off. Also when dry I get better traction.