Are all 8 spark plugs black (carboned up) like those 4, or is it just those 4? The pix of the 1st 2 may be oily, hard to tell. Some larger, higher res pix would help.
How many total miles driving on the engine at this point?
So the black stuff is not so much a worry now? Good. I'd check residual pressure in the system once it has set overnight. And the system will have to be heat cycled a few times again to get the air out now that you drained some coolant out again.
It helps some to keep the cap on loose while you warm it up and after you can see the coolant start to flow in the top of the rad, it'll purge some of the extra air. Top off the coolant at that point and close the cap down tight. With an overflow tank, it will take a few full heat/cool cycles to get all the air out of the cooling system. And if you don't have an overflow tank (you have not said if you do yet), then you need to fill the rad to 1 or 1-1/4" below the rad cap and keep it there.