Make the pork: Position racks in the upper and lower thirds of the oven; preheat to 350 degrees F. Tie the pork with kitchen twine at 1-inch intervals so it keeps its shape. Combine 2 tablespoons olive oil, the zest and juice of 1 blood orange, the garlic, rosemary, coriander, 1 1/4 teaspoons salt and a few grinds of pepper in a small bowl. Rub all over the pork. Set a rack in a large roasting pan; put the pork on the rack and let stand at room temperature, 30 minutes.
Meanwhile, quarter the remaining 3 blood oranges (do not peel). Transfer to a rimmed baking sheet along with the red onions. Drizzle with the remaining 1 tablespoon olive oil and season with salt and pepper.
Roast the pork on the lower oven rack until golden brown and a thermometer inserted into the center registers 145 degrees F, about 1 hour 10 minutes. About halfway through, roast the oranges and onions on the upper oven rack until softened and just starting to char, 25 to 30 minutes; set aside until ready to serve. Remove the pork from the oven and let rest 10 minutes.
Meanwhile, make the mostarda: Combine the blood orange juice, wine, 1 cup water, the raisins, sugar, honey, mustard seeds, rosemary and 1 tablespoon mustard in a medium saucepan; bring to a boil over medium-high heat, then reduce the heat to medium low and simmer until thick and syrupy, 30 to 35 minutes. Remove the rosemary sprig and stir in the remaining 1 tablespoon mustard and the vinegar. Season with salt and pepper. Set aside until ready to serve. (The mostarda can be made up to 4 hours ahead; reheat before serving.)
Untie the pork and slice; transfer to a platter along with the roasted oranges and onions. Serve with the mostarda.