I'm curious. Gonna go do an oil change on my E with a little over 5,000 miles on Mobil-1 full synthetic and pour it into a clear container just to see what exactly I have floating around in there. My guess is that it's just a tad bit darker than brand new and that there's nothing in the oil. Will report back tomorrow with pics.
Black is not bad, it shows that the detergents in the oil are doing their job. Back in the early 60's we used a straight weight non detergent oil and it was not uncommon for it to remain "clean" looking though the first few thousand miles or so. Detergent oils, when they became available, shocked many people as they would turn black within a few hundred miles. Your oil becoming darker as you rack up miles is not a bad thing.