Blood burn

Mab vaulted to the unicorn’s back with about as much effort as I used to fall into bed, and said, “ ’Ware,” before snapping her fingers.
All of her blood that had been scattered around, and the unicorn’s, too, abruptly went up into heat and light like flash paper. It left my face and part of my neck seared as if by a sunburn. Butters very briefly managed a Human Torch impersonation and whirled on Mab in shock, the skin of his forehead and cheeks and hands as red as if he’d had them soaking in hot water. “What? Why?”
“I warned you,” Mab said calmly.
“She can’t leave her blood lying around,” I said. “Corb and his people use magic. If they get their hands on it, it’d be bad.”
Butters frowned. He knew a lot more about how magic worked than most people. He was something of a neophyte at sorcery himself, and could manage a few fundamentals, and I could see him running through the possibilities of someone getting a magical handle on Mab through her blood. “Even her?”
“It is foolish for most to attempt to chain a tigress,” Mab said. Her wide eyes swiveled to me. “Yet chains can be forged—and tigresses can be caged.”
“Kind of a solid rule, man,” I said. “Doesn’t matter how big something is. If it bleeds, you can bind it.”
Butters surreptitiously examined his own hands, presumably for any leaking cuts.
Mab showed her teeth and said, “Indeed.”

Battle Ground Chapter 28, Page 260-261