In a small bowl, combine the chili powder, smoked paprika, garlic powder, 1 teaspoon kosher salt, and black pepper.
Place the chicken thighs in a shallow dish or bowl and add the spice mix, rubbing it into the flesh.
Cover the chicken with plastic wrap and refrigerate for at least 30 minutes and up to 8 hours.
Place the peaches, onion, and bell pepper in a shallow dish and rub with the olive oil.
Lightly oil the grates of a grill.
Preheat the grill to medium heat.
Place the peaches, onion, and bell pepper on the grill.
Cook until they have a light char and are slightly softened, turning once (about 5 minutes on each side).
On a cutting board, remove the stem and ribs from the pepper and cut into chunks.
Cut the onion and peaches into smaller pieces.
Transfer the grilled items to a high-speed blender along with the brown sugar, peach preserves, champagne vinegar, mustard, ginger, coriander seeds, and the remaining 1 teaspoon salt. Blend until smooth.
Pour the sauce into a large sauce pot and bring to a simmer.
Cook the sauce for 30 minutes, until slightly thickened and glossy.
Set aside 1 cup of the sauce to use for serving.
Add the seasoned chicken thighs to the grill and cook for 5 minutes on both sides.
Brush the chicken with a generous amount of the BBQ sauce and grill again on both sides until the chicken is cooked through and the sauce starts to caramelize (about 10 minutes total).
Transfer the chicken to a platter and serve with the reserved sauce and your favorite BBQ accompaniments.