Update.
After blowing a clog free from the black drain with my air compressor with the radiator hoses removed, I have since removed the water pump and stuck the business end of the hot pressure washer wand into each of the water pump holes in the block, I was able to blow out an incredible amount of very dark water out through the block drain hole.
Tipping the wand up and down did a very thorough job of flushing out both sides of the block without removing anything but the water pump and the block drain pet cock. After a good long while, the water finally ran clear.
