Gaithersburg, Md.--Black's Guide Inc. announced Wednesday that it has been acquired by Abry Partners L.L.C. for $21 million. Black's Guide is one of the nation's leading commercial real estate directory publishers. The company publishes its proprietary database of commercial real estate properties in 53 editions, which cover 20 of the largest U.S. markets. Its listings of more than 75,000 properties are distributed to more than 150,000 commercial real estate professionals. Abry is a Boston-based $1.6 billion private equity investment firm that invests exclusively in media and related businesses.