The reason why you lose your balance with your eyes closed or even in the dark, is because your vestibular system creates an internal horizon so that your body knows exactly where it is. If your vestibular system has become disturbed then your internal horizon will not be level. i.e. it will be tilted.
Listening to the radio this week I was interested and happy to hear them discussing concussion in sport, especially football. The discussion was around the massive change in concussion protocol at football matches in the past 10 years. In the past if a footballer took a knock to the head he was dragged to the side line, asked his name and his whereabouts and then sent back on to the field. Nowadays a player is pulled off the field, attended by a medico and not allowed to play for at least a week. At last awareness of the debilitating effects of concussion is rising