The Blues fell to an insipid defeat as the Seasiders sub decided it 15 minutes from time at Fratton Park in a low point for the season.
It took until the 65th minute for Kenny Jackett's men to muster a shot, which summed up their display which lacked in penetration and urgency for much of the afternoon.
It proved aÂ missed opportunity in front of a crowd of 18,403 with promotion rivals Sunderland and Luton playing out a draw at the Stadium of Light.
The first half proved the worst 45 minutes of the season with Pompey failing to muster a shot on goal.
The atmosphere at Fratton was flat with the Blues struggling to find any impetus and energy.
Blackpool slowly began to gain some confidence as the home side laboured and threatened with two dangerous crosses in 60 seconds in the 31st minute, first seeing Armand Gnaduillet fluff the ball before Liam Feeney flashed a shot past the post.
Brett Pitman, who was starved of service up front, whipped a dangerous cross in two minutes before the break which the on-rushing Curtis couldn't reach.
The muted applause on the half-time whistle was generous considering the quality of action served up in the opening half.
There were no changes at the break with much better demanded after the restart.
But the first shot from Pompey didn't arrive until the 65th minute after sub Andy Cannon fired over from the edge of the box after he replaced the ineffective Andre Green.
Ten minutes later the afternoon got worse for the Blues as Long fired home the opener after Armand Gnanduillet flicked on a cross from the left.
There was a late rally of sorts for Pompey as Brett Pitman flicked a header into Christoffer Mafoumbi's arms in the 85th minute.
Matt Clarke forced a save with a low effort on goal two minutes later, before Jamal Lowe fired over on the turn late on.
But it never really looked like Pompey would get a return on an afternoon to forget.