This question has been asked many many times before. (you should try the search function)
I believe the lights are dimmed to enable your eyes to become accustomed to the lower light levels should there be a need to evacuate. Once the decision to evacuate has been taken, all engines and the APU will be switched off leaving only emergency lighting in the cabin.
I believe the reason for leaving the blinds up is so that you can see any hazards on the outside of the aircraft. If for example the left hand engine was ablaze, it would be noted and people would escape from the right hand side.
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