Yes, I have applied a very thorough riffle test and discovered nothing. However, I thought it somewhat odd for Blaine's most expensive release to be unmarked. Additionally, the description of the deck online uses the term "Ultimate Marked Deck." Also, the deck is a little old so there is less fresh information to be gained.
Somewhat off topic, the deck contains no double backed card, also would expect in a more expensive deck.
It really depends on the deck's design. Double-backers aren't a standard feature by any means, and certainly marking isn't, either.
Blaine's only released two marked deck designs to date. One was intentionally, blatantly marked - a special "gaff deck" version of his black Split Spades Lions deck made under the Bicycle brand name as "Bicycle Split Spades." It was a deal he worked out to make gaffed versions of all three of the Split Spades Lions decks. The black deck was the "Mind Reading" deck (marked) in black, the "Transformation" deck (Svengali) in red and the "Discover Magic" deck (stripper) in blue. The black deck was of a type called "readers" in that they didn't require special knowledge of a code or system in order to be read - it was marked in plainly readable letters, numbers and suit symbols printed in a subtle but not-too-hard-to-spot location on the backs of the cards. Still, when they were discontinued, many retailers were offering them for as little as $5 a pack - not bad for a high-quality, commercially-printed pack of marked playing cards from a major printing company.
The other marked deck was the White Lions/Black Lions series - all of them were marked in a very subtle way, but it wasn't a practical marking system for card sharps/table play, as the marks required a lot of "real estate" of the card back to be visible in order to be readable - it was a positionally-based system rather than a symbolic system where all the marks appear in the same general location. It was good enough for a magician to use and not easily spotted without a riffle test. While Lions Series A, Series B and Black Label as well as the Black Lions regular and seconds were all marked using the same system, which was light years ahead of the readers described above in terms of deviousness and subtlety.
To the best of my knowledge, none of Blaine's other decks are marked. There's little reason to believe the Gatorbacks are, though you reported seeing them advertised as the "ultimate marked deck." Are you certain you weren't confusing it with another product, actually called the Ultimate Marked Deck? The UMD is a Bicycle deck made by two French magicians that's a reader deck done with incredibly subtle marks incorporated into the back design of the Bicycle Rider Back. It was made available in red and blue and was initially offered for $25 a pack. That price steadily climbed as the deck sold out and USPC announced they discontinued printing any decks with alterations of their trademarks, including the Bicycle Rider Back design, in order to protect the trademarks from counterfeiters, but there are stories that the guys who made that deck managed to get some printed in late 2015 and they became available again - at what price, I don't know, as I haven't bought marked cards in a while and still have my original UMD decks in both red and blue.
As this topic is more about marked decks than about playing cards, I'm moving it to the Magical Cardistry Bonanza.