Does blood in mucus indicate a head injury?

No: If it is a simple closed head injury and the nose is not hit, you typically shouldn't have blood tinged mucus. Of course, if the nose is part of the point of impact during the head injury, then you will have blood in your mucus. Even if you were to have a severe head injury causing an epidural or subdural hematoma in your brain, you might not have blood in your mucus.

Not usually: Dry nasal tissues, local trauma are more common causes. After a serious head injury, one can have spinal fluid or bloody fluid leakage from skull fracture.