Albion's hopes of snatching the second automatic promotion spot from Queens Park Rangers and Bristol City evaporated at Bloomfield Road.
Goals for mid-table Blackpool by Mike Sheron, Pedro Matias and John Murphy sentenced the lacklustre Seagulls to their second defeat in ten matches.
Substitute John Piercy's effort with seven minutes left was nothing more than a consolation.
The target for Mark McGhee's side now in the remaining five matches is to make sure of a play-off place.
Sheron volleyed Blackpool ahead in the 24th minute following a free-kick, his ninth of the season making amends for a glaring miss minutes earlier.
Blackpool, with captain Richie Wellens controlling affairs in the centre of midfield, were good value for their lead at half time.
Albion fell further adrift on 58 minutes, Spaniard Matias rifling in his first goal of a loan spell from Walsall.
John Murphy slotted his tenth of the season on 79 minutes to make sure of Blackpool's first win for four games.
Piercy reduced the arrears in the closing stages after he was set up by fellow substitute Chris McPhee.
Piercy was introduced in the first half in place of the injured Gary Hart, who was off the pitch receiving treatment when Blackpool opened the scoring.
Hart must now be a major doubt for Monday's home game against Rushden and Diamonds.
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