The Musketeers (21-12) will face the winner of the Creighton-Providence game in Friday's semifinals. It will be Xavier's fourth appearance in the semifinals.
Malcolm Bernard had 12 points and 10 rebounds for seventh-seeded Xavier, while Sean O'Mara had 10 points in a win that almost assures the Musketeers a bid to the NCAA Tournament.
Kelan Martin had 15 points to lead the second-seeded Bulldogs (23-8), who have never won a game in four Big East Tournaments. They were hoping to be a second seed in the NCAA Tournament but that could in doubt now.
Neither team led by more than two points over the final 6 minutes until the closing seconds.
Bluiett, who leads the Musketeers with an 18.0 scoring average, came up big in the second half going 5 for 9 from the field, including 2 of 3 from 3-point range.
Both teams struggled from long range with Butler going 4 for 19 and Xavier 5 for 15.
Xavier: The Musketeers came into the game having lost six of eight and both wins were over DePaul. The streak coincided with the loss of point guard Edmond Sumner to a season-ending knee injury.
Butler: The Bulldogs swept the season series from Xavier. ... Butler and Xavier have been in three leagues together — Midwestern Collegiate, Atlantic 10 and Big East. ... The Bulldogs committed five turnovers in the first half, all within the first 6 minutes.
Xavier faces the winner of the Creighton-Providence game on Friday in the semifinals.