Found this while looking for info about inks. From 1904, includes pretty much everything you never wanted to know about various inks and how they are/were made.
http://www.archive.org/stream/inksth...crich_djvu.txt
Not in the best format for reading, but interesting anyway.
Some of what I've found makes me wonder if some of the darker black OM backs are actually blue, which can be made nearly black. And one of the common dark blues can be lightened.
But the same argument that goes against the blue OM works there as well, if the ink of the darker ones is really a blue-black, we should see a lot more blue ones. Just as we should eventually find a few more blue ones if it's an actual error in the color used.
As a funny technical note, the computer monitor I have is a flat panel one. I nearly bought an OM on Ebay because it looked BLUE! Then I sat up to type and as the angle I viewed it at changed it got darker, until it was black. Now I make sure I'm viewing at a good angle when I'm looking at cards.......
Steve B