Virgin don't have a 'Policy' to keep window blinds shut. I think it's a decision made by the crew on the day. Most airlines favour closing blinds after take off to dim the respecive cabins as at night, most people want to sleep. It also prevents retina burn out when flying into sunrise and having an intense beam of light suddenly burning out your eye balls.
Far as I'm aware, window blinds should be open for take off and landing. It's the cabin lighting that should be dimmed at night so that your eyesight becomes accustomed to your surroundings
CB