There are Blu-ray Discs specifically engineered for long-term archiving and have BER guarantees (anomalous bit read per gb of data stored per year archived or something).
I somehow thought the company making the M-DISCs was dead but apparently Verbatim (and others) still sells BluRay discs labelled as "M-DISC" and they're compatible with many BluRay readers/writers.
Typically if the surface of the disc is gold and darkens with writing, it's an LTH disc.