Downing, 36, has penned a one-year deal with the Championship club.
He had initially joined Blackburn from Boro in 2019 but his contract expired at the end of last season.
Downing said: "I always wanted to come back since the last game of last season and told the manager that.
"I said that if it wasn't to be here then I'd probably have stopped playing and would have looked into something new.
"I had a really good time here last season, I really enjoyed it."