Echoing what has been said - In general (there are exceptions, but this usually holds true):
Blue Seals were what was used before USPC moved across the river from Cincinnati to Erlanger. These are from the old factory and were printed on the old machines.
Black Seals are what was used after the move. Printed in the new (current) factory on new machines.
There is likely a period in the middle where seals / tucks / decks were likely mismatched. If you had thousands of old "Ohio" tucks left over when you made the move to Kentucky, would you throw them all away? Na, you would use them. You would simply change the city name the next time you printed tucks. No reason to waste them. They may have started black seals before the move because they ran out of blue seals before that time, I wasn't there so can't tell you exactly how things went down.
There WAS a point in time during the transition period where there were some major quality issues on the new machines. Some people knew USPC moved, and the first deck they opened that said Kentucky had some problems. Instant bad reaction to everything NEW. This, topped off with USPC's decision to release the "Standard" Bicycle decks which most magicians dislike due to the tuck design, meant that there was a lot of hate towards the "NEW" USPC. This lead to people hunting for the elusive "Blue Seals" because "black seals are crap."
While there may have been quality issues during the transition period at USPC, those are LONG in the past. The transition happened... wait for it... over SEVEN years ago now. 2009 I believe. While there may have been a few months of issues, we have since had seven years of high quality decks coming out of USPC. And, to top it off. even though most places sell the "Standard" bikes, USPC still makes the "Classic" bikes that we all like.
There ARE a few small differences between old Blue seal decks and new Black seal decks, but those differences are small. The machines used to print, finish, and cut the cards are different. Some people who have LOTS of experience handling cards will say that they prefer the cut or feel of the old decks. Personal preference.
Most Gold Seal decks I have seen were part of Richard Turner's custom run. He had Bikes made using an alternate cutting method that helps make certain types of shuffling significantly easier. I believe that they used what was at that time a different stock than regular bikes. These you don't usually find out in the wild unless at a magic shop - they are tuned to very specific requirements that nobody but a card worker would ever care about.
I hope this helps!
// L