Forest travelled to Bloomfield Road as a team in form having collected four points from Chelsea and Southampton since the turn of the year.
However, a much-changed team were made to pay for missed chances as struggling Championship outfit Blackpool hit four past their top-flight opponents to reach to the last 32.
Cooper took the decision to rotate the entire team which started the win at Southampton earlier in the week, yet the Forest XI featured five players that have been signed either in the summer or January.
Despite the alterations, Cooper would have fancied his side to defeat opponents who had failed to win any of their last nine games in the Championship.
Although Forest went close through Neco Williams during the early stages, Blackpool went ahead after 17 minutes as Marvin Ekpiteta converted from close range after a goalmouth scramble.
Forest created the better of the chances ahead of half time with Emmanuel Dennis, Sam Surridge and Williams all threatening an equaliser, and failure to take any of those opportunities allowed Blackpool to run riot in the second half.
After good work from Jerry Yates, he teed up teammate Ian Poveda-Ocampo to slot home, and moments after Surridge had clipped the woodwork for Forest, Blackpool ended the game as a contest.
CJ Hamilton was provided with the space on the edge of the area to fire a low shot into the bottom corner, sparking jubilation in the stands as Blackpool moved closer to a first home win since October 22.
As the game drew closer to a conclusion, Yates capped off a stunning afternoon for the Seasiders, firing a shot in off the underside of the crossbar from close range.
While Forest netted a late consolation through Ryan Yates, only a small section of the travelling support reacted ahead of making the journey back to the East Midlands.