As far as I can tell, this isn't necessarily true. From Chris Severson:
If you find 2009 cards with “Made in Cincinnati” on the side of the box with a Black Seal, it might be a ‘Transformer deck’ (Blue Seal in disguise!) or they could be complete garbage. They took the extra boxes from the old factory and put Kentucky cards in them to save on box costs — kind of a crapshoot, if you don’t know what to look for.
The only definitive way to tell is by comparing them to a blue sealed deck.
There's so much hype around blue seals, but in reality, most readily available custom decks feel the same. A Bicycle deck from a local store probably won't handle as well, though they're nice for sure.
Fun fact: Red seals mean the deck was from/used in/made for a casino.