Blinds should be open for take off and landing for two reasons.
1. So that you, or a crew member are able to see outside during these critical phases of flight. You might see something such as a fire or fluid leak or anything else, which could be detrimental to the safe operation of the aircraft.
2. The interior lights should be adjusted to the ambient light levels outside. If you need to get out of the aircraft quickly your eyes don't need to adjust.
Neither of these things could be achieved if the blinds were down.