"Official" has nothing to do with canon. It just means a product is authorized by the owners of the property to be created and sold -- i.e. it isn't bootleg merchandise. That's got nothing to do with story content. Heck, the
Spock helmet with the flashing light on top was official merchandise. The jigsaw puzzles where Spock had lime-green skin were official merchandise. That didn't give them canon value. They're two unrelated concepts.
That said, there were ideas from the Blish books that did have influence on fan thinking, for lack of any opposing information. For instance, Blish was the one who first proposed 40 Eridani as Vulcan's home star -- which has never been made explicitly canonical, but was all but confirmed in
Enterprise's fourth season when Vulcan was established as 16 light years from Earth. He was also the first one to state that the show took place in the 23rd century, though it was probably
The Making of Star Trek's mention of that fact that locked it into people's minds.
Also, the Blish adaptations sometimes included deleted scenes not present in the episodes. For instance, the adaptation of "This Side of Paradise" included a deleted explanation that the spores on Omicron Ceti III were an intelligent telepathic life form making humans docile to serve as their hosts. I was surprised when I realized that scene wasn't in the actual episode, because I've always taken it for granted that it was there. I don't think that's the only such instance.