Yes, the big hitting with great strength on the head will cause the blindness. Your friend must be affected in the brain nerves which cause the eyes feel blurry at the vision. It is serious. If it is not treated immediately, the block in the brain will cause the blindness. You should tell your friend to go to have a full check and accept the treatment as soon as possible.
That is very serious. Hitting on the head is no joke. The strong impact may cause head trauma and in the meantime damage eye and vision tissue.
It might cause problems such as retina detachment, vitreous hemorrhage, optic nerve damage and so on.It is very common to have eye and vision problem as stated above after head injury. And any one of it will cause you feel vision blurred and even go blind. So no matter the hitting in head is serious or mild, your friend should go to hospital to have a detailed check, find out what damages caused to his/her eyes. Take medical treatment if needed to avoid any more damages.
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.