you are required by law to comply with ANY reasonable request by the crew onboard your flight.
BlueTUI, I disagree.
I think you will find that you must comply with any LAWFUL/LEGAL request, not any REASONABLE request. I would contend that closing the blind is not a LAWFUL/LEGAL request.
I contend that a comparable situation would be when someone reclines their seatback to the detriment of the person behind. It may be reasonable to ask the person to put the seatback more upright, but it not necessary to comply - the seat is fitted with a recline mechanism, it is acceptable to use it. The window is fitted with a blind, likewise it is acceptable to use it - in either direction.
Edited to add this reference:
Every person in an aircraft registered in the UK shall obey all lawful
commands which the commander of that aircraft may give for the purpose of securing the safety of the aircraft and of persons or property carried therein, or the safety, efficiency or regularity of air navigation. (ANO Art.67)