Place the tenderloins on your work surface and begin to remove the silver skin (the thin, translucent, sinew-like covering on the outside) by using a knife to trim it away from the meat. Place the tenderloins in a baking dish.
Place the coffee, salt, brown sugar, paprika, pepper, garlic and onion powders, cumin, chili powder, and jalapeno relish in a small bowl and stir to mix. Rub all over the tenderloins, patting it onto the meat with your fingertips. Let the pork marinate in the refrigerator, covered, for at least 4 hours or as long as overnight.
Add butter to a cast iron skillet; sear the tenderloins 2 minutes per side.
Add blueberries and peaches directly to skillet and place in the oven to roast at 350°F for 15-20 minutes. To test for doneness, insert an instant-read meat thermometer into the meat: The internal temperature should be about 145°- 160° depending on desired doneness.
Transfer the grilled pork to a cutting board and let rest for about 3 minutes. Once the meat has rested it is ready to be sliced
For the sauce:
In a heavy sauce pot, sauté the onions in the butter until they begin to brown, then add all of the dry ingredients and cook for 2 minutes while stirring. Add all the liquid ingredients and blueberries; bring to a boil, reduce to a simmer and cook for about 10 minutes. Purée the sauce and strain through a fine chinois (strainer).