Today's vintage "Dynasty" article comes from 1984 when Diahann Carroll (49 at the time) joined the "riches-and-bitches soap" and People Magazine called her "the only black actress with a continuing role on a prime-time serial."
Carroll said many times (in interviews and documentaries) that she was fascinated by nighttime soaps (which were all the rage at the time), but she could also see a clear void - she wanted to become (as she herself put it) "Dynasty's" first "black bitch."
Carroll first appeared in the final two episodes of season 4 (aired on May 2 and 9, 1984) and it seems the network had already ordered 29 episodes for the following season by that time as the article mentions Carroll was under a lucrative contract for 17 of them.
The article also debunks the rumor that was floated at the time - that Dominique Deveraux was the major domo's daughter Kirby's long lost mother. Kathleen Beller, who played Kirby, was actually on her way out of the series.
Dominique, of course, did not reveal her identity in season 4 - we learned who she claimed to be in the concluding moments of the 4th episode of season 5 (which premiered October 24, 1984), so the writers had a couple more months to decide who Dominique Deveraux was going to be.