The visitors could have doubled their advantage when Weiss robbed Rankin and whipped a tempting cross from the right into the path of Jelavic but the striker was just too slow to connect with the goal at his mercy.
The visitors could have doubled their advantage within four minutes of the break when Marc Twaddle headed Healy's cross into the path of McLean, whose effort was blocked by Langfield.
Blackburn doubled their lead after 18 minutes, Roberts scrambling home from close range after Brett Emerton's cross rebounded into his path off Ryan Shawcross.