The local derby should prove a baptism of fire for the centre-back but he has already had experience with Leicester City and Derby County in the division.
Keane has been away with the England Under-21 squad but was withdrawn from the team named to face Wales close to kick-off. The youngster's brother Will is currently with QPR on a similar basis.
Rovers boss Gary Bowyer has been hunting a new defender due to a spate of injuries at the back and the 21-year-old has joined for the remainder of the campaign, wearing no.16, the same jersey his namesake Roy had at Old Trafford.
"He is highly sought-after," Bowyer told Blackburn's official website. "An England international at Under-21 level.
"He has had a good education with Manchester United and he has got experience already in the Championship with his time on loan at Leicester and Derby - two top clubs.
"We are delighted to have been able to secure his services until the end of the season."