It's possible jazz (or piano bar) source songs/music used in films as old as, say, Casablanca, The Killers (the Rozsa one) and others, becoming more and more acceptable, influenced the main film music itself... Hollywood had always needed time to accept new music forms, especially what was considered as "black" or "sex" music.
Just got this CD (I know!!). It's pretty good, isn't it (rhetorical). I was never a fan of Goldsmith's CHINATOWN (don't like the film and the score leaves me cold. I have the LP but rarely played it, outside of the occasional spin to see if things have changed...they haven't). But this and Fielding's BIG SLEEP hit home for me more, in this jazzy/noir music score genre. I haven't fully digested MONKEY SHINES yet, but FML is great to get to know. And far be it for me to argue with the exceptional sequencing credentials of David Shire and Lukas Kendall, but I'm not sure about that Dixieland type cue (Three Mile Ltd) coming right after that BIG brash suspense moment/finish (Take Me To Your Lido). But then again, I'm likely to drop the source cue when I send things over to my on-the-go player. But still...bloody good stuff.
I think the solo instrument (trumpet or sax, usually) over a drowsy and smoky jazz backdrop might have originated in the 50s with TV detectives--the solo instrument representing the crusading private detective and the slow jazz indicating late-night vice and shady characters.
A Mike Hammer series in the 80s used Earle Hagen's evocative "Harlem Nocturne" as its theme, but I believe there were some shows from the 50s. While Barry used an alto sax, and Goldsmith a trumpet--Shire's Farewell My Lovely uses a trombone--and it's perfect.
BTW: Harlem Nocturne is such a great tune, it sounds great on any instrument.