Posted By:
Tim NewcombNo information we have uncovered indicates that any "300 Series" cards exist with blue backgrounds. My own beliefs, which I hope to substantiate convincingly in print before we're all dead and gone, are:
--that the blue background cards were the earliest to be printed, and then after issuing the first two series, the decision was made to switch to the pastel backgrounds.
--that the 300 Series backs were the last series to be printed. The last group of new cards (Series 19-24) are found exclusively with these backs, and then the same "300 Series" plate was used to reprint some of the earlier series (or possibly team sets) that were in continued demand. Since Series 1 contains so many HOFers and stars, it was in great demand, which is why Series 1 players with 300 Series backs are found more often than players in other series-- but all with pastel backgrounds.
No evidence that the two groups overlap. If someone can show different, please let us know!
More in the eventual article 
Tim