Wall Street might be disappearing into a black hole, but the stars are aligning for Warren Buffett.

The veteran value investor has swooped in on Constellation Energy Group, the natural-gas and power company that lost more than half its value this week. MidAmerican Energy Holdings, part of Berkshire Hathaway, has thrown Constellation a lifeline in the form of a tentative $26.50-a-share takeover offer.

Constellation,...