SL Green Realty sold a 49 percent stake in Stonehenge 57, its 261-unit rental building on Sutton Place, to asset manager BlackRock in a deal that values the 19-story property at $170 million, the landlord announced Wednesday. In late 2014, SL Green bought a 50 percent stake in the residential and commercial portfolio owned by Stonehenge Partners, which included 2,800 rental apartments and 43,000 square feet.