For the potatoes: Peel the potatoes and cut into 1-inch chunks. Place the potatoes in a large saucepan of cold, salted water. Bring the water to a boil over high heat. When the water boils, lower the heat to medium and simmer the potatoes until they're fork tender, 15 to 20 minutes.
For the venison: Season both sides of the venison tenderloins with salt and pepper.
Heat a large saute pan over high heat and add the oil. When the oil is hot, but not smoking, add the tenderloins. Reduce the heat to medium and sear the tenderloins on all sides, 3 to 4 minutes per side. Remove the loins from the pan and let rest until ready to serve.
For the potatoes: Drain the potatoes well and transfer to a bowl. Mash the potatoes until there are no lumps. Stir in the butter, nutmeg, cream, salt, and white pepper. Stir until creamy, adding more cream if needed. You want smooth, creamy potatoes. Taste and season with more salt and white pepper, if needed.
For the sauce: In a small saucepan, combine all of the ingredients, except for the blueberries and butter. Bring the mixture to a boil over medium-high heat. Lower the heat, add the blueberries, and simmer until reduced a little and thickened, 8 to 10 minutes. Remove from the heat, strain the sauce back into the saucepan, season with salt and pepper, and then whisk in the butter. Hold warm until serving.
To plate: Spoon potatoes into the center of the plate. Slice the venison loin into 1/2-inch thick slices on a bias. Lay slices over potatoes. Top with sauce.