Preheat oven to 425 degrees F. Pat chicken thighs dry using paper towels.
In a small bowl, combine the salt, garlic powder, and black pepper. Sprinkle evenly over both sides of the chicken thighs.
Heat a cast iron skillet or other heavy oven safe pan over medium heat until hot and add olive oil. Carefully arrange the chicken in the pan, skin side down, and cook without moving the chicken for 7-9 minutes, until the fat on the skin has rendered and the skin is nicely browned and crispy.
Flip the chicken over and top each thigh with a slice of blood orange. Arrange the rosemary sprigs on top and transfer the pan to the preheated oven. Roast for 18-20 minutes, until the chicken thighs are cooked through and reach an internal temperature of 165 degrees F when tested with an instant read meat thermometer.
Remove the chicken from the oven and transfer the chicken thighs to a plate. Keep warm. Drain any fat in the bottom of the pan, then place on the stove over medium heat again. Add the sliced onion to the pan and cook, scraping up any fond (leftover browned bits of chicken) from the bottom of the pan. If the pan is looking dry, add a tablespoon of olive oil. Cook for 8-10 minutes, until the onions have softened and begin to caramelize.
Add the blood orange juice, chicken broth, honey, rosemary, and black pepper to the pan and simmer until the sauce is reduced by half and a slightly thicker glaze remains, about 6-8 minutes. Taste and season with salt, if needed, then return the chicken to the pan and spoon the glaze and caramelized onions over the top to serve.