Preheat oven to 375°F. In a small bowl, whisk together honey, rosemary, olive oil, smoked paprika, salt and black pepper. Set aside.
Heat a large skillet over medium-high heat and sear the pork tenderloin on all sides. Place seared tenderloin into a large baking dish and spoon honey mixture on top. Place into the oven and let cook, uncovered, basting with the juices at the bottom of the pan every 10 minutes, until a thermometer inserted into the thickest part of the tenderloin reads 155°F (68°C), about 35 minutes. Remove from oven and let the tenderloin rest for at least 5 minutes before slicing.
Prepare sauce while pork is in the oven. Heat a drizzle of olive oil in a medium pan over medium-high heat. Add shallot and garlic, and sauté for about 5 minutes. Add blueberries, sugar, white wine vinegar, water and cayenne pepper, then reduce heat to medium and let simmer for about 15 minutes. Remove from heat and transfer into a blender. Add grainy mustard and purée mixture until smooth. Season to taste with salt and pepper and serve with sliced pork tenderloin. (The sauce will keep for up to 1 week in the refrigerator.)
Total cook time: 45 min