Marriott claims that there are no blackout dates for points redemption by Platinum members. I tried to book the Fairfield in Moab/Utah for a stay in September. Rooms are available, but not for points redemption. For other dates, points redemption is possible at the same property. So, there may be no blackout dates, but quotas for rooms on points. And if you only set the quota low enough.... same effect like blackout dates.