Making backups is really a game of probability. Assuming the data is successfully written to any media (as confirmed by the backup program's "verify backup" function), the weak link becomes the shelf life/survivability of the media. Backup tapes can break and be demagnetized. Hard disks can crash. Optical media (like DVDs and Bluray disks) degrade over time. I view the question of "Is it safe to back up to media X?" less of a yes or no question and more one of your goals and retention requirements.
If you're looking at a one-time/one-off/adhoc backup that you plan to use in the short term for recovery, then it's less an issue of reliability and more a question of convenience.
Assuming that you're looking at a corporate server backup solution (e.g. ongoing backups, some media rotation schedule and some retention period requirement for each backup), it's still less of an issue of reliability (since you'll assumingly have at least a daily backup) and more one of convenience. So assuming your backup process is rigorous (done according to a schedule and verified for errors) and frequent, I see no issue taking advantage of the larger capacity of Blu-ray disks.
Under no circumstances though would I rely on any optical media for long term storage. For long term, tape will be most reliable. To really reduce risk of long term backup storage and avoid restore failures I think it's important to have multiple backups stored in different locations.