Joseph Baratta, Blackstone’s global head of private equity since 2012, is preparing to leave the firm, according to people familiar with the matter. The nearly three-decade Blackstone veteran is one of the firm’s most senior executives, serves on its board and sits on its management committee. The timing has not been finalized but is likely to fall around the end of this year. His potential departure would be the latest among senior leaders to leave the private-investment giant.