Going through the new FAQ page I found this under the heading "What are Blackout Dates"... interesting to me because this seems to me to be a new and stronger outlining of the policy, in our favor, or was this always here and properties could avoid it if they wanted to. I know we have all experienced "point rooms not available" dates that skirted the "no blackout" policy.
Frequently Asked Questions | SPG & Marriott Rewards
Cut and pasted from the above link, bold is by me:
What are blackout dates?
Blackout dates traditionally refer to a limited number of dates, such as during peak travel times throughout the year, when a hotel could choose not to have Members redeem Points for a Free Night Award stay. With our “No Blackout Dates” policy, if there is a standard room available at a Participating Property for booking by any means, whether cash, credit or Points, then you can reserve the room using Points.