Are you sure it is blood and not just red colored? It may be that she simply had an upset stomach and threw up her stomach acid and some food stuffs. As long as she is eating and drinking well, it shouldn't be too much of a concern. When she starts acting lethargic and less playful, as well as if her eating and drinking habits change, you may consider taking her to the vet. Dogs will eat grass as a means to make them throw up or pass food more quickly to help calm an upset stomach.
This answer was edited by justins86 26 days ago.
Reason: addendum
