Answer:
The blood is from her sons neck injury by the poisoned darts.
Step-by-step explanation:
Sherlock Holmes have tried to make this case really interesting by keeping suspense and thriller at the peak. The adventure of Sussex Vampire is a short story which is full of suspense. Watson arrives in Sussex at the request of Mr.Ferguson to identify the real culprit for the blood sucking for his younger child. The real culprit turned out to be Jack who was half elder brother of the victim and he used to throw poisoned darts to kill his younger brother because of his jealousy. Mrs.Ferguson used to suck blood from the neck of her younger son to save him from death.