Although Matt Oakley gave the Rams the boost of a 27th-minute lead, a missed penalty by Steve Howard proved to be the turning point as Blackburn hit back with goals from Roque Santa Cruz and David Bentley.
The visitors should have wrapped the match up in the second half and deserved to take the three points but Derby will look back on the penalty incident which could have changed the course of the game.
When Ryan Nelsen tripped Kenny Miller, Howard stepped up to drive the spot-kick low to Brad Friedel's right, but the keeper got a hand to the ball and dropped on the rebound before Eddie Lewis could turn it in.
Within a minute Blackburn were level, and when Bentley fired them in front, there was no way back for Paul Jewell's struggling side.
Derby are now ten points adrift of safety and, although Jewell plans to bring in at least five new faces in January, their plight already looks hopeless.
Blackburn are at last moving in the right direction, but their defence again looked suspect and they will have been unhappy with the build-up to Oakley's opener.
A cross from Tyrone Mears went past Nelsen allowing Oakley to fire under the legs of Friedel from ten yards for his third goal of the season, but first since August.
After Howard's penalty miss, Blackburn recovered with Santa Cruz scoring his eighth goal in five games after Benni McCarthy's shot had been parried by Lewis Price.
It got worse for Derby when Aaron Mokoena won a challenge with Oakley and Bentley burst forward to drive a low 25-yard shot inside Price's right-hand post.
Price kept Derby in the game after the break with good saves from McCarthy and Santa Cruz while Bentley saw another left-foot shot strike a post.
Derby kept going and won two corners in stoppage time, but Blackburn held on to record only a second victory in their last nine Premiership matches.