Both Trailblazer engines will run fine on 87 as they were designed too, but there are benefits to running 91 like smoother running and more overall power. The Trailblazer has a static compression ratio of 10.0:1 and under full boost (22psi) that can increase to as high as 18.0:1. When using 91 the timing maps stay on the high end which delivers the most power while running on 87 pulls timing to prevent detonation which also reduces power. I always got slightly better gas mileage on 91, but since it's also more expensive it won't offset the cost. I suggest experimenting with both octanes to see if you notice any difference.