These films dealt with the topic, but I'm not certain is a kiss occurred.
Island in the Sun (1957)
From IMDB:
"As rumor has it an interracial screen kiss caused quite a commotion in the U.S. when the film was released."
It's been so long since I saw this that I can't be certain if one does occur. I'm still looking for verification.
Hiroshima Mon Amour (1959)
It's the same with this film. I'm still trying to find out if it's a yes or no. The romance is a Japanese man and a French woman.
The World, The Flesh and The Devil (1959)
There are sexual overtones between Inger Stevens and Harry Belafonte, who also is in "Island in the Sun". I don't think that there's a kiss, but I do know that Harry Belafonte was considered a "sex symbol", far more than Mel Ferrer.
Guess Who's Coming to Dinner (1967)
IS there a kiss? It's been so long since I've seen the film that I simply can't recall. Nowhere does it refer to a kiss. By the way, her name is spelled "Katharine", like her aunt's name; she's Hepburn's niece. In turn, she's actress Schuyler Grant's aunt!