Blackstone, based in New York, is adding retail properties as rents and occupancies rise. Shopping-center vacancies dropped to 10.7 percent in the fourth quarter from 10.8 percent in the previous three months and 11 percent a year earlier, according to New York-based data provider Reis Inc. Effective rents, or what’s paid after any landlord discounts, averaged $16.59 a square foot, up from $16.51 a year earlier.