Finely chop one of the peaches; add ground turkey, salt, and pepper. Shape turkey mixture into four 1/2-inch-thick patties (if necessary, dampen hands first). Grill on the rack of an uncovered grill directly over medium-high heat for 4 to 5 minutes per side or until no pink remains (165 degrees F). Add cheese; cover and cook 1 minute more.
Meanwhile, coarsely chop remaining peaches. In a large skillet combine peaches, blueberries, and chili powder. Cook, stirring occasionally, over medium heat for 5 to 6 minutes or until heated through and juices are beginning to form.
Top each piece of garlic bread with one turkey burger and some of the peach mixture. If desired, top with mint and additional chili powder.