Brian from Backblaze here. We can't find them either. We buy either 4 TB or 8 TB right now, and we're experimenting with 10 TB drives. We don't know why the 6 TB drives appeared and then disappeared...
Are those 8TB and 10TB the helium ones with HAMR? Very slow.
My understanding of HAMR is that it is probably perfectly fine for Backblaze's backup products, which are (more or less) write-once, read-rarely. Shingled magnetic recording should also be OK for that use case.