^Well, at the time when the first Blish novelization would've been written, Paramount hadn't yet bought Desilu, so it would've had to be a licensing deal between Bantam and Desilu. Novelizations of TV series were pretty common at the time, even more so than today (perhaps because TV sets weren't so ubiquitous and there were few reruns), so it might've been pretty routine for a studio and a publisher to make a tie-in deal for a new show. I'm not sure who would've initiated the deal, but at the time ST began, Desilu was considered a washed-up studio, with nothing in production beyond The Lucy Show; commissioning Star Trek and Mission: Impossible (and later Mannix) was part of a strategy to rebuild Desilu into a force to be reckoned with. So I'd guess they would've probably been the ones to approach publishers about tie-ins to help promote their shows. But I could be wrong.
And Bantam wasn't the only licensee. They got the license for the novelizations and published the first one in 1967, but the following year we got the young-adult Mission to Horatius from Whitman Books and the nonfiction The Making of Star Trek from Ballantine (which went on to publish other ST nonfiction/behind-the-scenes books in later years, as well as the TAS adaptations by Alan Dean Foster, all while Bantam retained the license for TOS fiction).
