Rowntree, 19, son of former Reds player Kevin, joined Premiership Blackburn as a schoolboy and was playing reserve team football when he suffered a knee injury that kept him out for 15 months. He was released by Rovers last month and played two games during a week's trial with United where he was known by youth team coach Eric Kinder from his time at Blackburn.
The native West Cumbrian impressed in those two games but Blues boss Neil McDonald chose not to offer him a contract, McDonald commenting : "I thought Gari did all right but I'm looking for a little bit more. Ziggy (Aranalde) is our main left-back and then we have got James Krause who has played a couple of games, done well and is working hard at his game with us. With Ziggy on nine bookings, James might get another chance soon."