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 Frank Fielding the wrong way with his ninth-minute spot-kick.
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 goalkeeper recovered to make a save.
Sandomierski then made a superb save at his near post to keep out Chris Martin's header on the half hour mark. Back at the other end, Grant Hanley headed David Dunn'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.