Yes, blackout dates do apply for both Stay Plus FREE Nights and all Accor+ rates.
You can check the blackout dates here: https://all.accor.com/loyalty-program/bod/index.en.shtml?msockid=380e1f60cdae637600500ba6cc9c62cb
Just search for the hotel you'd like to stay at and enter your dates to see if there are any blackout periods.