BlackRock went all in on Wall Street’s booming business of private lending last July when it acquired HPS Investment Partners, a firm founded by alumni of Goldman Sachs that was one of the stars of the sector. Days after the deal closed, an analyst at HPS’s Midtown Manhattan headquarters spotted a big problem. The company was the lead lender on a more-than $400 million credit agreement with a telecom entrepreneur, Bankim Brahmbhatt , accepting as collateral accounts receivable the executive’s firm had acquired from other businesses.  Reviewing those invoices, the analyst noticed that the email address domain on one didn’t…