The Blues were undone by the goal-kicking of centre Christian Lealiifano, who kicked the Brumbies to a 20-13 Super Rugby win at Eden Park.
As well as conceding penalty points, the Blues were unable to hold their passes at crucial stages, possibly owing to an increasing sense of desperation the further the game went on. Defeat left their play-off hopes looking forlorn, especially with their tour of South Africa still to be undertaken.
Lealiifano kicked five penalties, but failed to convert Nic White's first-half try. The Blues, scoreless in the first half, hit back in the second period with a penalty from Chris Noakes and a well-worked try from Rene Ranger.
However, the Brumbies weathered the home side's fight-back to earn a hard-fought victory in Auckland.
The Cheetahs gave their play-off hopes a major boost with a 34-22 bonus-point victory over the Southern Kings in Port Elizabeth.
Four tries in a 23-minute period either side of half-time blew open the match with Johann Sadie twice touching down.
Piet van Zyl and Willie le Roux also went over while Elgar Watts kicked 14 points as the Cheetahs firmed for a wild-card place after the Blues, Waratahs and Crusaders all lost this weekend.