The unofficial answer: BLACKOUT dates are those dates where only cash is king as per MR terms and conditions, ". . . add blackout dates, limit rooms available for any Reward at any participating hotel . . .", in other words, you can't use points or other special deals such as a free night certificate, etc. - only cash. NO ROOMS AVAILABLE means that even if you want to use the cash-only option, the property is booked to capacity and no rooms remain.