Jordan Rhodes scored his 26th Rovers goal of the season from the penalty spot to give the hosts an early lead.
Scott Dann's header in first-half injury time ensured Blackburn's first Championship win at Ewood Park in just over two months and condemned Derby to a 12th away league loss of the campaign.
Blackburn were given the opportunity to make the perfect start when Jeff Hendrick brought down Rhodes inside the penalty area.
The Scotland international picked himself up before sending Fielding the wrong way with his ninth-minute spot-kick.
Rovers had their tails up after the early goal and Josh Morris fizzed a shot wide before Frank Fielding smothered another Rhodes effort.
A dreadful error by Grzegorz Sandomierski midway through the first half presented the ball to Derby's leading scorer Jamie Ward on the edge of the box but the Polish keeper recovered to make a save.
Back at the other end, Grant Hanley headed David Dunne's corner against the crossbar.
It was from another corner, this time taken from the other side by David Jones, that Blackburn gave themselves a two-goal cushion.
Fielding came for the cross but the ball slipped through his grasp and Dann was on hand to nod in from close range.
Rhodes had a glorious chance to make it 3-0 in the 62nd minute when he was one-on-one with Fielding but the Derby keeper made a good save.
Enjoying decent possession, Conor Sammon had tried a shot from outside the area in the first half and did so again after 70 minutes, and the result was the same as he missed the target.
Jamie Ward did a little better with 10 minutes remaining, forcing Sandomierski to get down to his left to make a save after bursting into the box.