Seconds, traditionally, are usually not the top-notch, perfect decks, but decks with minor defects that aren't enough to affect game play. As HankMan said, it might have a slight error to it like a slightly incorrect shade, or maybe the faces are more off-center than usual, something along those lines. Seconds are still considered of acceptable quality - anything with a serious defect would be considered too bad to sell.
In this case, it could have been a preliminary print run that had the wrong shade of blue or something and USPC balked at eating the cost of the reprint - an issue that was enough for Blaine to consider it to not be what he wanted to make, but not enough to be called a "defect," per se. It could even be just the tuck boxes that are not 100% up to what he wanted rather than the cards. (Or it could be an interesting way of marketing more cards than he normally would be.) I'm sure they'd still be desirable.
