Bradley Johnson returned to Pride Park to inspire Blackburn to a 4-0 victory and condemn Derby to their worst start for 28 years.
Johnson, who left Derby in July 2019, scored twice as Rovers ripped County apart with Tyrhys Dolan and Adam Armstrong also finding the net.
It is the first time since 1992 that Derby have lost the first three games of a season and the defeat could have been heavier but for David Marshall who made a string of good saves.
Derby had started strongly with Max Bird seeing a shot cleared off the line but Blackburn stunned them with three goals in the space of four minutes.
The home side were caught out by a rapid break in the 11th minute and after Ben Brereton's shot came back off a post, Dolan tapped in the rebound.
Derby were still reeling from that when Johnson made it two a minute later with a 25-yard drive that fizzed into the bottom-left corner of Marshall's net.
Marshall could do nothing about Johnson's second in the 15th minute when another strike from distance took a big deflection on its way into the roof of the net.
Rovers looked like scoring every time they went forward and Brereton almost added a fourth in the 20th minute when his back-post header was saved by Marshall.
Derby's defensive vulnerability was exposed again in the 33rd minute as Joe Rothwell surged into the box but his attempt to pass the ball into the net resulted in a simple save for Marshall.
The Scotland international rescued Derby again when Lewis Holtby played in Armstrong but Marshall made a brilliant save.
Derby had strong appeals for a penalty turned down and Louie Sibley had a header tipped over before Blackburn almost added a fourth on the break at the end of a remarkable first half.
Derby switched formation for the second half but Blackburn still created chances with Brereton firing over from a good position before Rothwell ran into the box but again failed to find the power to trouble Marshall.
Both sides were finding space and Kamil Jozwiak got in on the right in the 65th minute but could not get enough bend on the ball to find the far corner.
Derby had another chance seven minutes later when Nathan Byrne's cross from the right was met by Lee Buchanan but he headed over from eight yards.
Any doubts about the outcome were removed in the 77th minute when Harry Chapman broke clear on the right and set up Armstrong for a close-range finish - his sixth goal of the season.
What the managers said...
Derby's Phillip Cocu: "Last season we had two bad games and I compare this a little bit with those games because I think today a big part of our defeat was our attitude, our passion, willingness and desire to defend and track down. You can talk about tactics but in any system it's about how you execute it and if you concede three goals in four minutes, I think that says enough.
"We showed some good football but I am worried about how we present ourselves when we don't have the ball and that's a big part of the game. We had 65 per cent possession, so what do we do when they have the ball? It's not good enough, that's underperforming.
"We always say stay in the game, for a few minutes get yourself together, take initiative and go forward again but by conceding three goals and two similar goals, it's unacceptable. In the second half, the attitude was much better but it's a little bit late when you are 3-0 down and that's what I'm upset about because I expected we would have learnt from those games of last season."
Blackburn's Tony Mowbray: "The players deserve huge credit for their work-rate, their effort and energy they are putting into the games and are getting the rewards for all the hard work we've done over the last six months when we've tried to change the team around a bit.
"I think the benefits are there for all to see. I'm told we had 23 shots so to score four, I think it's been one of our issues, the percentages. We've worked a lot on finishing and the final pass into the box and picking the right pass when we get into crucial areas.
"That was something that was letting us down, we were able to dominate games but not finishing teams off because we weren't taking chances but I don't think you can be too greedy. For Johnson to score, that's fantastic for him. He's growing into this football club a little bit now, he looks as if he's irreplaceable at the moment, in the last four games anyway, so let's see if we can maintain it and Bradley can maintain it."