City could have taken the lead on 10 minutes but Leon Best mis-timed his header from David Bell’s cross.
Forest went ahead on 42 minutes following a quick break 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 Keiren Westwood.
A brilliant save from Lee Camp to deny Clinton Morrison at the start of the second half kept Forest in front.
Blackstock doubled 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.