Joseph was probably born in that part of Prince William Co., Virginia, that became Fauquier County (1759). Joseph's parents were William and Elizabeth (Crump) Blackwell. His marriage to Ann Grayson Gibson probably occurred in Fauquier Co., Virginia. She was the daughter of Jonathan and Susannah (Harrison) Gibson. Four children by Ann have been identified, and she died around 1800. In 1802, Joseph Blackwell remarried to Mary Waddy Brent in Fauquier County. She is said to have been the daughter of William and Hannah (Neale) Brent. Four children by his second marriage have been identified.
Joseph inherited 700 acres by his father's will (1772) and is said to have been granted over 5000 acres of land for his service in the Revolutionary War. He "entered the gallant 3rd Regiment of Virginia Continental troops as a cadet with James Monroe, and served throughout the war," including the battles of Harlem Heights, Princeton, Trenton and Brandywine. He was taken prisoner at Charleston, South Carolina. He was "repeatedly promoted," becoming a lieutenant of the 6th Virginia Regiment, until he attained the rank of captain. At war's end he returned home to Fauquier County, where he continued service in the 85th Regiment (apparently militia) in which he rose to the rank of colonel.
He is further said to have been a justice of the county court and twice to have qualified as high sheriff of the county, a position held apparently until his death. Joseph died, presumably at his Elk Run home in Fauquier Co., Virginia in 1823, and a laudatory obituary was published in the Warrenton Spectator, which ended: "No man of his county was more useful; none whose loss will be more severely felt in the neighborhood in which he lived." (Bio by Mark Hale)