Wayne Rooney's first match in permanent charge of Derby ended with a 1-0 defeat at home to Championship relegation rivals Rotherham.
The former England captain was officially confirmed as the Rams' manager on Friday, having held the post on an interim basis since the departure of Phillip Cocu in November.
But his reign got off to an inauspicious start as Jamie Lindsay's late strike sent County second bottom of the table.
Rotherham moved ahead of their opponents on goal difference and they also have two games in hand on the teams around them.
The standout news from Rooney's team selection was his decision to hand the captain's armband to 19-year-old Jason Knight.
Lee Buchanan drilled an effort towards Blackman's right-hand post after 26 minutes, but the Chelsea loanee got down quickly to turn it behind for a corner.
Rotherham thought they had taken the lead 10 minutes before the break. Captain Richard Wood applied a fine finish inside the area but, after a brief delay, Wood was ruled offside and the goal was chalked off.
The second half was an even more cagey affair, with chances at a premium.
Martyn Waghorn fluffed a rare opportunity when well-placed after 70 minutes, firing wide of goal from just a couple of yards out.
The game livened up as it headed into the latter stages and Shinnie again tested Blackman from close range before Waghorn's looping header almost caught the keeper off-guard.
But, after soaking up pressure, the Millers snatched victory against the run of play in the 86th minute.
Shortly after having a goalbound effort hacked away by a back-tracking defender, Lindsay knocked home the winner from a few yards after Daniel Barlaser's free-kick had crashed against a post.