No blackouts does not refere to the number of rooms available for points purchase at a hotel. Blackouts are periods the hotels had no rooms available for points purchase period. Unlike Starwood (if they have a room you get it) Marriot still alots a certain number of rooms per night that are available for points purchase. This is similar to the airlines where they only have so many seats per flight that can be purchased for airline points.
In the past certain properties were not available at Christmas or during some peak event at that location. Now you will have to be online making the reservation right when the dates are vailable so that you get one of the limited number of rooms available.