Steele joins Rovers on a season-long loan after he was told he did not figure in Aitor Karanka's plans for the season.
And Chelsea youngster Blackman has been brought in until January 17 as back-up to current No 1 Tomas Mejias and Dimi Konstantopoulos.
Blackman has represented England at every age group from U21 to U19 and recently signed a new long-term contract at Stamford Bridge.
As for Steele, Boro's first-choice keeper until the start of this season, he will now challenge Paul Robinson for a place in Blackburn's side.
Boro boss Karanka said: "I said if he has a good choice to go then for me it wasn’t a problem. I told him one or two months ago if he had another choice he could go. He knows that he is not my first choice or my second choice."
Boro have also sent Seb Hines on loan to League One side Coventry City until January 4.