with a Dahua NVR and Dahua Cameras it has an Automatic Network Recovery feature that will retrieve footage off the cameras automagically and fill in any gaps in timeline that it can..
BlueIris does not have this functionality, but you can always go in and manually download the video off the camera if it captured something while your NVR was down..
Perfect Example happened to me a year or so ago, I took my NVR down at midnight to install a 6TB WD Purple in it.. I have the NVR sitting in my lap when I hear my home automation system announce: The Mailbox was just accessed, I look over at my display of all my cameras and see my PTZ zooming in on some asshole in my mailbox.. Called the cops and they were here in 2mins as there were hundreds of mailboxes that got broken into at the time, but I had absolutely no recordings because I was attempting to expand my recording capabilities... Murphy is an asshole like that.
You dont need really big cards, usually a 32GB is enough for 24h of h265 and thats enough for most backup purposes, any catastrophic failure you should be aware of within that timeframe.