Jeff Blackburn is retiring. The head of Amazon’s Global Entertainment Group will sign off from the company in January after more than two decades with the retail giant and streaming platform. The moves were announced Friday via internal memos from Amazon CEO Andy Jassy and Blackburn himself. The news comes a year and a half after Blackburn returned from a yearlong sabbatical in a new role as head of the company’s global entertainment group that included combined oversight of music, audio entertainment, games and video for the company. Blackburn’s decision to retire comes after a busy week at Amazon. The e-commerce giant this week fully integrated MGM into its exec ranks after its $8.5 billion purchase of the storied studio closed in May. Amazon Studios head Jennifer Sal...