Derby confirmed the move on their official website, snapping up the striker on a three and a half year contract.
Earlier this week it had appeared that Blackman was heading to Sheffield Wednesday, with SkySports reporting the Owls had a bid accepted by Reading for the striker.
The in demand forward played for rivals Sheffield United in the 2012/13 season before moving to Reading, and it is unknown if this played a factor in him snubbing a move to Sheffield Wednesday.
He can point to Derby's league position as being key in his choice to join the Rams, with Paul Clement's side currently in the automatic promotion spots in second place, with Wednesday outside the play-offs in seventh.
There is more competition for places up front at Derby, and he may well have received more playing time at Wednesday. Time will tell if he has made the right decision, and gets a chance to play Premier League football next season.
From Wednesday's perspective it is disappointing, but the interest in Blackman had come as a surprise, and there is plenty of time left in the transfer window for the club to look at alternative targets.
Here is a look at how Owls fans reacted to the snub...
Have to be honest and say there are better, long term proven options than Blackman out there. Not loosing sleep over that one #swfc— King SWFC Badger (@kasey1581) January 6, 2016
Shame Blackman has gone Derby, but a great breath of fresh air to be competing with the top performing players for once #swfc— Tom Magness (@tom_magness) January 6, 2016
Disappointing to lose out on Blackman but have to accept Derby are a better bet for promotion and so had an advantage.... #swfc— Steve Craine (@stevecraine) January 6, 2016
Can't blame Blackman at all. Wish him all the best. Any fan that claims 'didn't want him anyway' is a complete fickle idiot. #swfc— TommyCraigStringer (@TommyOwl) January 5, 2016