Leaving the blinds open at night ensures the procedure remains consistant and doesn't rely on the staff to decide whether to open or close. Besides, it might be morning by the time you're rescued!
There`s not CAA requirement for the blinds to be open for take off and landing. There is a requirement for blinds in or next to the doors to be open so that if there is an emergency, outside hazards (engine fire etc) can be accessed before evacuation. Some airlines probably just have an in-house rule about it.