New Mountain Capital is selling its stake in accounting firm Citrin Cooperman to an investor group led by Blackstone, the first flip of an audit firm by a private-equity investor. 

The deal, which would value Citrin Cooperman at more than $2 billion, is the latest in a series of stakes that private-equity firms are taking in midsize audit and consulting firms in the U.S. Accounting firms have been hungry for more money to pay for technology and talent. Many private-equity funds, meanwhile, consider accounting firms low-risk investments that could net a sizable return based in part on recurring revenue streams.