For the blackening spice: In a small mixing bowl, combine the pepper, cumin, granulated onion, cayenne, granulated garlic, Italian seasoning, paprika, chili powder and salt. Mix to blend. Store in an airtight container if not using immediately.
For the garlic butter: In a medium saucepan over medium heat, add the butter and garlic, and cook until the butter has melted and garlic has become fragrant, 5 to 6 minutes. Mix in the parsley. Reserve to the side in a small bowl.
For the donkey sauce: In a small mixing bowl, combine the mayonnaise, roasted garlic, mustard, salt, Worcestershire and pepper. Mix well. Cover and reserve.
For the black and blue burger: Preheat a grill to medium-high heat.
Divide the ground beef into eight portions; roll each into a loose ball, then flatten into a 4-inch patty. On four of the patties, place 2 ounces of the blue cheese. Cover with a second patty and gently seal the edges to form a stuffed patty about 1 1/2-inches thick.
Season the stuffed patties on both sides with the blackening spice. Grill, spaced 3 inches apart to prevent flare ups, until a crust has developed on the first side, about 3 minutes. Flip, and cook to develop a crust on the reverse side, 2 minutes. Top each burger with 2 slices of bacon and 1 ounce of the remaining blue cheese. Cover with a piece of foil and cook another 30 seconds to melt the cheese. Remove the burgers to a serving tray to rest.
Lightly brush the cut sides of the brioche buns with garlic butter, and toast on the grill until crisp and golden, 5 to 10 seconds.
To assemble: Smear the buns with some donkey sauce. Layer the bottom buns with pickles, onions and a burger. Top with tomatoes and lettuce. Cover with the bun tops and secure with wooden skewers. Serve.