So, I figured out the problem 
I didn’t check the inputs quite as thoroughly as I should have, and upon further inspection discovered that some adjustments I made were subtly messing the normalization up. Everything is working fine now.
On the plus side! If anybody is trying to work with generated images instead of files, or wants to pass a ready-made source in to a DataBlock, it turns out ItemGetter(i) does the trick perfectly for both get_x and get_y. And if you just format your data as ImageTensors, the DataBlock handles it like a charm.