Finally found a scarce 4th edition Blue Book with a black cover. The only Blue Book ever bound in another color other than blue: I have a 1st edition Blue Book in the normal blue cover plus a 3rd edition in a robin's egg blue cover, and now this 4th edition with a black cover:
Nice! I have a sizable library of books, many first editions and hardcover as I'm a purist. Physical books still rule in a digital age.
The third edition came in all sorts of shades of blue. No one really knows why there are some black 4th editions. Being that the 3rd and 4th editions were printed in 1944 and 1945, I think World War II had a lot to do with supply of paper and cover materials available at the time.