Well, the hairball may have grazed his esophagus a bit and cause a little bit of bleeding - like when sometimes we blow our noses too hard a bust a small capillary, which heals almost immediately. I suggest you observe your cat for the next couple of hours. If he is not in distress (pain, vomiting, no appetite, depressed, meowing, etc.) then, the spot of blood may have been from a small abrasion caused by the expellation of the hairball.

However, you should still take her to the vet as soon as possible; she may have eaten something sharp. If it gets serious, I do believe there are emergency vets you could call.