Up 20-7 at halftime, the Blues lost their shape as the Brumbies found the chances to get their feared lineout maul rolling for two tries to get back to 20-19 down with five minutes to play.
As first five-eighths Noah Lolesio lined up a 40-metre dropped goal attempt with a minute to go, it was charged down by replacement prop Ofa Tu’ungafasi and in the resulting play the Brumbies were penalised.
That saw the Blues tap and clear the ball at fulltime to secure a place in their first Super Rugby (full season) final since 2003. That was also achieved in a semifinal against the Brumbies.
The Blues seemed comfortable heading for the final in the first half in front of 26,900 fans who braved wet and cool conditions.
Replacement hooker Lachlan Lonergan scoring twice while the Blues suffered crucial yellow cards to hooker Kurt Eklund and flanker Adrian Choate.
But torrid defence minimised the time available to the Brumbies in the final count.
Captain and first five-eighths Beauden Barrett again demonstrated outstanding control and marshalling the side. He went close to scoring a try but knocked the ball on in a tackle over the line, but his most important play was when Brumbies fullback Tom Banks looked set to score.
Racing for the line he was denied by a flying Barrett tackle in the corner.
However, the Brumbies had proximity and benefited from time in the Blues 22m area when scoring a 57th minute lineout maul try to replacement hooker Lachlan Lonergan.
It reduced the margin, but a failed conversion by first five-eighths Noah Lolesio meant they still required two scores to win.
Then, in the 70th minute, the Blues conceded a turnover penalty that allowed another maul to be launched before it resulted into another penalty for a collapsed maul. But Robinson contested the lineout, secured the ball and the Blues won a penalty.
Superior discipline in the first half had reduced the Brumbies' chances to employ their lineout maul from close to the Blues' line. On the one occasion they did pressure the line, they several times had the ball held up and, finally, referee Ben O'Keeffe ruled there was no chance to continue and the Blues were able to clear the line.
The Brumbies stormed in for the first try of the game after two minutes when, from a scrum, second five-eighths ran straight at Stephen Perofeta acting as stand-off, getting through him and then carrying on in what was almost a slow-motion play.
However, the Blues regathered themselves, Perofetaespecially to land two penalty goals before he ran the ball back and with contributions from halfback Finlay Christie, centre Rieko Ioane, the ever-present Barrett and finally it was No8 Hoskins Sotutu who crossed for the Blues' try response 22 minutes into the half.
It seemed every time the Blues were able to lift the tempo of the game they made substantial progress. Inevitably, lock Tom Robinson was involved in gaining good ground before the ball was let loose. Barrett backed up a strong run by Rieko Ioane with a run into the goalmouth before the ball was worked through Perofetato win Mark Telea who scored to give the Blues a 20-7 lead at the break.
The Brumbies were unable to consistently break the defensive shackles and were only able to make 110metres compared to 276 by the Blues. By comparison the Blues were able to break 19 tackles to only four from the Brumbies. The Blues also dominated the rucks 33-19.
It was the Blues on the receiving end of tackles in the second half, finishing the game making 111 to 91. But as it has been for much of the season, the defence paid dividends in the end.
Scorers: Blues 20 (Hoskins Sotutu, Mark Telea tries; Stephen Perofeta 2 con, 2 pen) Brumbies 19 (Irae Simone, Lachlan Lonergan 2 tries; Noah Lolesio con). HT: 20-7
The Blues host the Crusaders in the DHL Super Rugby Pacific final at 7.05pm on Saturday 18 June at Eden Park.