I don't think there was much segregation of the music as much as the record labels were major and independent. A lot of the black artists were on the independent labels so got less exposure and less likely to be hits. A good example is Nat King Cole; a black artist on Capitol who got multiple hits all through the 50's and into the 60's. Some even called the iconic Capitol records tower in LA "The house that Nat built."
This was the earliest KIOA Survey I could find on line - from 1957. Chuck Berry, coincidentally is in the number 3 position with School Day(s). Notice most of the hits WERE on the major labels of the day. So, in my mind, there was no discernible prejudice on the radio for pop/rock at the time.
http://las-solanas.com/arsa/surveys_item.php?svid=205
I first became aware of top 40 radio when our family got a console stereo for Christmas 1964. Being a naive Iowa boy I had no idea which songs were from black artists and which were from white artists. I would maybe learn by seeing them on Ed Sullivan, Hullabaloo or Shindig. People were generally aware of 'Motown' as a major force in soul music of the time but I don't remember much other than that. Again, I was pretty young.
If anything, I'd say the 80's were more segregated. There was black/disco/rap and white heavy metal and pop with some crossover (Basically Michael Jackson and Prince) but the stations played more specifically along the lines of race than I remember in the 60's.