I had failures of both "archival" quality DVDs and SD cards before, just after a year or two after last backup.

Do you have any links on the methodology used to measure the 30+ or 1000yr grade for a blu-ray ?

Blu-rays are but a few years old. It's preposterous for any one to claim that they are "guaranteed" for 30 years, let alone 1,000 years. Not to mention that that guarantee most likely has an asterisk that limits damages to the cost of the original blu-ray disk in case it actually fails.