Elites generally have no blackout dates except for some properties that don't participate.
The following properties either do not participate in or do not fully participate in “No Blackout Dates” at this time:
This was copied from the Marriott Rewards Terms and Conditions
Elites do have problems with blackout dates. The no blackout dates policy is misleading. That being said, a call to the help desk can sometimes shake a room loose from a property that limits points rooms.
There are a few threads on here that discuss this very topic. Type "blackout dates" in the search box.
Not sure about the Marriot, not ever been so high up in the status due to various technical reasons.
Being a Diamond member of Hilton from last few years, I can answer the second part. Yes, I never had issues with the blackout dates in my reward bookings.
Like the suggestion from the other friend. Try the support/help desk. Might be you will get lucky.
If there are 'events' in or around a particular area (like the Super Bowl in Indianapolis or the Indy 500 in Indy) there will be no availability for points usage. They sell out the hotels and will 'block' the property from points usage. Can't blame them. It's cash sales that pay the bills. But, as painedplatinum said, their no blackout dates policy is misleading.
Hotels have standard rooms available for redemptions every day. Blackout dates traditionally refer to a limited number of dates on which a hotel could choose not to accept redemptions. With our new “No Blackout Dates” policy, hotels will no longer have blackout dates for redemptions. Hotels may limit the number of standard rooms available for redemption on a limited number of days.