Not true that there is any sort of official rule to this effect. I've slept in a lot of crew hotels that had only semi-decent curtains.

It is true that well-behaved airlines, who take an active interest in fatigue prevention, do try to book hotel rooms with blackout curtains and good sound proofing. This is however not a universal behaviour and not all destinations where layover is necessary have any hotels with these qualities.

In searching to attempt to find corroboration for this factoid, literally the only things mentioning pilots and blackout curtains I could find were your own comment here and a random forum post from a pilot saying blackout curtains don't really work. Nothing about any requirement that pilots only be placed in hotel rooms with blackout curtains.