Shape ground beef into 8 thin, 4-inch-wide patties. Mound 2 tablespoons of crumbled blue cheese on each of 4 patties, leaving a 1/2-inch border. Cover each with one of the remaining patties. Crimp and seal the edges closed. Sprinkle with blackened seasoning.
To grill: Lightly oil the grill rack. Grill the stuffed patties over medium-high heat, about 4 minutes per side for medium-well. (Be sure not to press the burgers as they cook or they'll split open and the cheese will ooze out.)
To broil: Cover a broiler pan with foil and coat with cooking spray. Broil the stuffed patties in the upper third of the oven, about 4 minutes per side for medium-well. In either case, let the burgers stand for 5 minutes before serving.
Serve with crispy bacon, lettuce, tomato, sweet onion and bleu cheese dressing.