Blu-ray is fine. Will it last 20 years? Probably not. Some of the CD-Rs I burned on my first 4x writer are still readable, but things change. Regardless, if you're not putting all your eggs in that basket, it's still a convenient way to store and catalog data for easy retrieval. I envision something where each disc is labeled "raw footage from episode #327" or whatever. If the disc fails, fine, go to the HDDs. It's inexpensive to add an internal (or USB) reader to any computer, assuming it doesn't already have one, so accessibility is good.
As to HDD reliability, I have a whole stack of 40 and 80GB drives. One developed bad sectors last time I tried to read it (about a year or two ago.) Others still work and hold, for e.g., Hackintosh HDD images (because it's a laborious and painfully slow process to reinstall). If I lose them, ... eh.
I have 4 drives (RAID-5) in a NAS. When I replace those for bigger capacity drives, the old 4 become backup drives for the new ones. NO RAID on the backups -- you don't want the hassle of trying to arrange for an appropriate read environment later on. A USB-to-SATA cable or dock will be available for at least as long as the drive is viable. This process of upgrade and trickle down keeps each generation of drives "fresh" within 2-5 years. I plug them in every six months or so to backup new data. (It really should be more often, but that's a risk I take.) IMO, this is a decent compromise between leaving them at rest, and periodically checking for faults.
Since video archival won't be updated, doing a checksum validation of backed-up data once a year would suffice to catch drive faults or media read errors before both the BD-R and HDD have had a chance to fail. Could a simultaneous failure still occur? Yes, of course. That's life. Dave might be out one episode of raw video then. I suspect he'll survive.
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