With the sale of an office building in Boston late last month, Blackstone Group Inc. closes its books on a $39 billion acquisition of Equity Office Properties Trust, the largest real estate transaction ever. Blackstone made the 2007 deal near the top of the market, and for a long stretch during the economic downturn it looked like a disastrous move. The company at one point wrote down its investment to less than 70 cents on the dollar, even though it offloaded some of the riskiest office properties right after completing the...