John was the son of Francis Blackwell and Elizabeth Cornell. He studied under Rev. Eaton in Hopewell and received holy orders from the Hopewell Baptist Church in 1764. In 1782, he took oversight of Upper Freehold Baptist Church. He owned Glen Moore Mills, a general store. He was taxed in Hopewell Twp., old Hunterdon (now Mercer) Co., 1779-81. He was readmitted to the Hopewell church in 1789.