Blackwell School National Historic Site Act This bill establishes the Blackwell School National Historic Site in Texas as a unit of the National Park System to preserve, protect, and interpret for the benefit of present and future generations the Blackwell School, its role as both an academic and cultural cornerstone in the community in Marfa, Texas, and its function within a segregated system of education in Texas and the United States from 1885-1965. The historic site shall not be established until the date on which the Department of the Interior has (1) entered into a written agreement with the Marfa Unified School District providing that the Blackwell School shall be donated to or placed by agreement into comanagement with the United States for inclusion in a national historic site to be managed consistently with the purposes of such a site, and (2) acquired sufficient land or interests in land within the boundaries of the historic site to constitute a manageable unit. Interior may only acquire by donation, purchase with donated funds, or exchange any land or interest in land located within the boundary of the historic site. The bill prohibits any private property or nonfederal public property from being included within the boundaries of the historic site or managed as part of such site without the written consent of the owner. Interior shall prepare a general management plan for the historic site.