Yes, it is possible to get temporary blindness given rise to by the hit on the head. According to what you have described, your friend feels vision blur, which is also named double vision, caused by damage to nerve. In other words, it is a nerve dysfunction caused by blood hydroncus due to external injury invading the nerve. Your friend needs to receive a treatment to diminish the asepsis inflammation caused by the blood hydroncus without delay and to activate the nerve to regenerate the repair nerve, otherwise once the ischemia nerve denaturalizes, it is impossible to recover. You had better ask your friends to check the magnetic resonance of his skull and receive the treatment as soon as possible. Wish your friends recover quickly.