Blackstone Group’s second-quarter results rebounded as the surging stock market lifted the value of the private-equity giant’s portfolio.

The New York firm swung to a profit after posting a steep loss in the first quarter as the S&P 500 recorded its biggest percentage gain in more than two decades and finished up 20% in the period. The value of its private-equity portfolio climbed by 12.8% compared with a 21.6% decline in the first quarter.