When I Spy was first conceived by Sheldon Leonard he wanted one of the characters to be black, Leonard at first wanted the character of Scotty to be a 25 percent partner and Kelly was supposed to be a 75 percent presence on screen. Robert Culp told Leonard that it had to be 50/50 or the comedy wouldn't work and Leonard immediately agreed. Same thing happened with the spy show The Man From U.N.C.L.E. as the character of Illya Kuryakin was only supposed to be a minor sidekick to Napoleon Solo, but the producers on that show realized that wouldn't work either. Both shows became huge hits at the time.
I watched I Spy originally when I was a kid. Growing up in Southern California having basically the first black actor starring along side of a white actor I don't remember a single person thinking this was controversial at the time, at least not in my neighborhood, the San Fernando Valley. Everyone just thought that Scotty and Kelly were cool spies. Every kid wanted to be them. When they didn't want to be them they either wanted to be James Bond or the guys from the Man From U.N.C.L. E. I Spy was better because the characters of Alexander Scott and Kelly Robinson were more realistic than say Napoleon Solo and Illya Kuryakin. Show has aged well over all these years and still great.