Blackstock made all three points safe in the 78th minute when he blasted the ball into the roof of the net from 12 yards.
Victory ensured Forest ended 2009 unbeaten in 16 gams - the club's longest run without defeat in the league since the Brian Clough era when they went on a record-breaking 42-game unbeaten streak between November 1977 and November 1978.
Coventry made the short trip across the midlands on the back of their longest winning run for just over two years after victories in their last three matches.
And they should have taken the lead after 10 minutes but Leon Best mis-timed his header from David Bell's cross.
At the other end, Kelvin Wilson went close with a header before Keiren Westwood had to tip an effort from Earnshaw over the crossbar.
Forest went ahead following a quick break led by Radoslaw Majewski. From a Coventry corner, the Pole collected the ball and raced downfield.
He found Earnshaw with a perfectly-weighted pass and the Wales international expertly lifted the ball over the advancing Westwood.
A brilliant save from Lee Camp to deny Clinton Morrison at the start of the second half kept Forest in front.
Nathan Tyson had a chance to make it 2-0 soon afterwards but he dragged his shot wide from a good position inside the box.
Blackstock did double the Reds' advantage with 12 minutes remaining. Dele Adebola did well down the left-hand side of the box and crossed the ball for his strike partner to shoot past Westwood.
Adebola went close to a third goal in the final minute but his shot from the edge of the area just cleared the crossbar.