Preheat oven to 375 degrees F (190 degrees C).

Season lamb generously with salt and black pepper. Heat oil over medium-high heat in an oven-proof skillet. Sear lamb on all sides until well browned, 3 to 5 minutes per side. Turn off heat. Remove lamb to a platter.

Mix together Dijon mustard, rosemary, cinnamon, and cumin in a small bowl. Spread a thin layer of the mustard mixture on each rack of lamb.

Place chopped shallots and pinch of salt into the same skillet. Cook and stir over medium-heat until shallots begin to lose moisture and become browned and soft, about 5 minutes. Add blueberries and continue cooking for about 2 minutes. Stir in red wine, turn off heat, and stir mixture to deglaze the pan.

Place the racks of lamb into the skillet and interlace the rib bones together to push the lamb racks close together, leaving a lamb bone "zipper" in the center. Place the lamb racks into the center of the skillet. Season with a pinch of black pepper.

Place skillet in pre-heated oven and roast lamb until internal temperature reaches 125 degrees F (52 degrees C) for medium-rare, about 20 minutes.

Remove lamb to a cutting board to rest about 5 minutes.

Drizzle balsamic vinegar into the blueberry sauce; add a pinch of salt. Turn heat to medium-high and bring to a simmer to reduce sauce by half, about 4 minutes. Turn off heat and stir in butter. When butter is melted, check seasonings and add more salt if needed.

Slice racks into pieces of 2 ribs each. Serve topped with a spoonful of the blueberry sauce.