Ken Frazier, the longtime chief executive of Merck and one of the few remaining black CEOs of a Fortune 500 company, is retiring. Frazier, Merck’s CEO for the past decade, will be replaced by Rob Davis, the chief financial officer, the company said Thursday. Frazier will become executive chairman of the board during a transition period. Frazier joined Merck in 1992 as general counsel to one of Merck’s pharmaceutical businesses and worked his way up to the top job. He is one of the few black CEOs at the head of a Fortune 500 company. Last month, when Walgreens named Roz Brewer as its new CEO, there were four. With Frazier’s departure, that number is back down to three. Frazier was instrumental in orchestrating some of Merck’s most successful ventures, including the acquisition of the cancer drug developer VelosBio. The VelosBio deal was aimed at expanding Merck’s cancer drug franchise, …