I'm not certain what your question is. Do you want to kill the Blood Grass, or weeds growing in the Blood Grass patch? If the idea is to kill weeds, without harming the Blood Grass, I'd suggest you apply the chemical directly to the weed, and not spray it on. Use a cotton glove over a surgical glove, and spray the glove with the chemical, then rub it onto the weed without touching the Blood Grass. Another approach is to use a foam paintbrush as the applicator.

That said, I think I'd be more inclined to pull the weeds, rather than poisoning them. I don't know if Calcium Acid Methanearsonate kills Blood Grass or not, but I wouldn't want to experiment with it.