The difference between the black back and this shade of blue is so subtle that it doesn't surprise me at all that these cards weren't discovered 50 years ago. Plus, lots of people have been looking since the Walsh was discovered and Powell will only be the second one found (if it checks out). So, that makes this doubly not surprising because there are so few of them. These are not neon blue backs with 10,000 examples floating around, they are nearly black backs with two floating around - not hard to believe that they were only recently discovered. Also, don't discount the role of the internet and thousands of scans for comparison at your finger tips -- this was not possible until recently.
Note, I did say "if it checks out". The reprint card that people mention is the Cy Young here:
http://www.net54baseball.com/showthread.php?t=156986
That reprint has a pretty good back. There are also lots of reprints with cruddy blue backs that would never be confused with this one. The question is whether someone can take a blank backed T206 (one that is factory cut too --- something that I am not aware of existing), go through the whole lithography process to create a perfect match for an Old Mill back, and then print it on a few cards.
There may be a way to mimic lithographic printing that would allow for the printing without needing to create a stone, etc. (I will leave that to the printing experts - I have no idea). It may also be possible to remove the ink from the back of a factory cut card and then print over that, I am not wise in the dark arts of card alterations.
Ted -- to me the blue looks darker than Piedmont blue. Ron - can you do a side by side with a Polar Bear too?