Pat the duck breasts dry, and season generously with salt and pepper. Place in air and water tight bags (vacuum or zip bags). Make sure all the air is removed. Place in water bath at desire internal temperature. See notes below
Remove duck from the bags. Pat dry thoroughly.
Place breasts skin side-down in heavy-bottomed 12-inch non-stick or cast iron skillet and set over high heat until sizzling, about 2 minutes. Reduce heat to medium and cook, moving and pressing breasts to ensure good contact between skin and pan until golden brown and crisp, about 5 minutes. Flip and cook second side until barely colored, about 30 seconds. Transfer to paper towel-lined plate, cover with foil, and allow to rest while making the sauce.
Remove all but about a tablespoon of duck fat from the pan. Add the minced shallot and garlic, and saute about a minute over medium high heat. Add cumin, and saute another minute or so.
Deglaze the pan with sherry, scraping any browned duck bits. Stir in the juice and zest, chocolate, agave nectar (if using), and bitters (if using). Season.
To Serve: Slice duck into ½" thick slices. Top with the blood orange chocolate sauce, and garnish with cilantro.