James Simpson-Daniel on the way to the second of his three tries
Gloucester face Northampton in the LV= Cup final after gaining revenge over Cardiff Blues for their defeat by the Welsh team in the 2009 final.
The Saints beat Saracens 21-10 in the day's second semi-final.
Gloucester trailed 10-3 until Freddie Burns struck shortly before the break and James Simpson-Daniel went on to claim a hat-trick in the second half.
Ex-Blues fly-half Nicky Robinson kicked nine points and Blues tries came from Casey Laulala and Ceri Sweeney.
The final takes place on Sunday, 21 March at Worcester's Sixways ground.
The Cherry and Whites lost 50-12 to the Blues in last year's Anglo-Welsh final and suffered a 26-25 defeat against them this season's pool stages, but were not to be denied again.
The sides were locked at 10-10 at the break, ex-New Zealand centre Laulala going over for the Blues while full-back Burns crossed for the Cherry and Whites. Blair and Robinson also kicked five points apiece.
But a hat-trick from Simpson-Daniel put the visitors out of sight, meaning Sweeney's late try was little consolation for Dai Young's side.
Some ill-advised adventure from the home side allowed Gloucester to level the scores on the stroke of half-time.
Sweeney looked to move the ball wide with a cut-out pass but it was picked off by Burns who sauntered over and Robinson converted.
Gloucester carried that momentum into the second half and after Ben White was sin-binned for slowing down the ball, they moved the ball right after a quick-tap penalty for Simpson-Daniel to score and Robinson converted from the touchline.
The Blues responded with a spell of pressure of their own and Blair kicked his second penalty to narrow the deficit to 13-17.
But that was as close as the hosts got as flanker Akapusi Qera fed Simpson-Daniel for his second, and a Burns chip then gave the ex-England wing his treble.
Good work from Laulala put Sweeney in for a try two minutes from time. But it was not enough for the Blues, who now face a battle for Heineken Cup qualification through the Magners League.
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