Blairstown is a township in Warren County, New Jersey, United States. As of the 2010 United States Census, the township's population was 5,967 reflecting an increase of 220 from the 5,747 counted in the 2000 Census, which had in turn increased by 416 from the 5,331 counted in the 1990 Census.The area had been known as Smith's Mill and was later called Butts Bridge, named for a family that owned the eponymous crossing of the Paulins Kill.Blairstown was incorporated as a township by an act of the New Jersey Legislature on April 14, 1845, from portions of Knowlton Township, based on the results of a referendum held that day. The township was named for John Insley Blair.