The Blues will meet either Western United or Koonibba in the Far West football preliminary final after opening the finals series with a 19-point success against Thevenard on Saturday.
The Blues started well and kept the Magpies at bay for the remainder of the contest as they won 10-8 (68) to 7-7 (49).
In blustery conditions at Penong it was the Blues, who finished fourth on the table, who got the jump in the first quarter as they kicked four goals to one Thevenard behind to lead by 25 points at quarter-time.
Three consecutive goals meant the Magpies had reduced the gap to five points late in the second term yet crucially the Blues added two quickfire majors to lead by 17 as the siren sounded.
Two early goals from Thomas Clayton and Brendan Kloeden saw the Blues carry that momentum through to the third term and they proved the only majors to see the lead increase to 31 points by the final break.
Yet Thevenard were not about to let their season end without a fight as they kicked four goals in the final quarter.
The Magpies twice kicked back-to-back goals and at one stage got to within 14 points, only for Clayton to boot decisive goals which helped the Blues advance.
Clayton finished with four goals as Kloeden and Matthew Grimshaw added two apiece, while Robin Saunders grabbed two for Thevenard.
The first grand finalist will be decided this weekend when Western United meets Koonibba, with the loser to take on the Blues.
A dominant final quarter helped the Blues progress by 41 points over Thevenard in the B grade.
The Blues pulled away late on to win 10-3 (63) to 3-4 (22).
Dylan Zippel was pivotal with four goals and was the only player to bag more than one.
There was a lone major in the first quarter which went the way of the Blues, who pulled clear in the second term with three goals to lead by 23 points.
Thevenard – who managed a solitary point in each of the opening two quarters – hit back with with the only three goals of the third term to see the gap shrink to three points.
Yet the Blues took command in the final quarter with six goals to no score meaning they will face either Koonibba or Western United.
Koonibba edged past the Blues in the under 17s with a 5-5 (35) to 4-4 (28) victory.
Leading by 21 points at three-quarter time looked to give the Roosters the edge, however a scoreless final term made for a thrilling climax as they just held on to win by seven.
Kevin Penny kicked four of their five goals.
The Blues cruised to a 91-point win against Thevenard in the under 14s match.
The Blues were never troubled as they won 14-8 (92) to 0-1 (1).