Thanks for all the tips!
When I did the tests, the ammonia, nitrates and nitrites were at zero. Hardness was hard and PH was neutral. The test strips cost 11 bucks (so 22 for the 5 in 1 and the ammonia strips that can do 25 tests). If the liquid kit really does a hundred tests hands-down I am getting that.
The guppys were not from wal-mart. The only fish I ever got from Wal-Mart was a dragon goby that was being kept in a goldfish and koi tank. I would give them more chances, but normally the fish appear to be on the verge of death there.
I have had two guppies give birth and both of them were a lot thinner than the two that died before birth. My moscow blue female was still fat, but not like a minature manatee. She was also "box-shaped" like a guppy should be before birth. She only had 8 fry which might explain the thinness. My moscow black was bigger overall, but still thin for being pregnant and never got the box shape. She had the babies without really any signs of her being that late into her pregnancy. The second time, she got much fatter than the first time. Same with my half-black that died. That could just be a cooincidence though.
Judging by all the responses, and the hardiness of the fry compared to the adults, I am going to draw the conclusion that it was just stress and a bacterial infection that killed the guppies. Hopefully soon, I will have enough that I can breed my own guppies without having to transfer any from stores to my house (which will calm them down) and the guppies will be distant enough cousins that they will not die on me.