A general area deployed EMP strong enough to wipe out data in HDD, we will have a much much bigger problem than losing data in HDD. The HDD head is so close the disk platter, even a small power magnetic pulse can alter the HDD content. But with a EMP deploy say by military weapon, the chance it has effect on HDD is minimal .
Just for reference, HDD magnetic head size to platter distance is like a 747 flying 2 feet off the ground. When I was in the HDD read head industry, we have to develop mechanism to handle the event when the head make contact with the platter.
But I do agree, BlueRay, or any optical disk media, the shiny part of the disk is metal. They are subject to oxidation. I have lose many disk due to this. I certainly won't archieve anything with an optical media.
The most reliable achieve media I have seen is magnetic back up tapes. I have seen my previous company's IT recover data a magnetic tape with water and physical damager simply by boiling the tape and roll it on to a new take holder. I was very impressed. But this isn't a very doable method for most people.
Between cost and reliable and convenient trade off, I think HDD is the way to go these days. This may change in future.