The 20-year old Norway international will stay at Ewood Park until January and is excited about furthering his development in the Championship as opposed to with United's reserves.
"Why waste six weeks of my career in the reserve league for United when I can develop myself and become better at Blackburn?" King told TV2.
"Henning (Berg) came with an offer I could not refuse. Now I look forward to playing myself into the team, score a few goals, and help Blackburn up the table."
Rovers boss Berg added: "Joshua is a good player who will give us a physical presence, pace and a more rounded squad.
"He will get the chance to prove himself, and regain more match fitness with us the next week."