Do you think it is reasonable? I am sure if you asked someone walking down the street, that if he/she was asked to close a window blind by his boss, to aid in a potential cooling of a room, what would be his/her response?
As far as I understand it, after all passengers have disembarked the outgoing crew are obliged to carry out a chck of the aircraft which involves visiting every seat.
So the instruction(?) to close the blinds is not a great imposition as they will be close to them anyway.
Personally I tend to look at such minor things with the view of "If it's not illegal or dangerous" my employer has every right to request me to do it during work/duty times.