"The Blu-ray system reduces costs by 50 percent and energy use by 80 percent compared with its current cold-storage system, which uses hard disk drives.."

Not bad. Anyone know how it would compare to tape?

Tape is still cheaper per Gb. Google uses StorageTek SL8500 and Spectralogic T-Finity libraries to manage it's backups and cost is significantly smaller then trying to do that with spindles.