To answer your question: No, you can't trust blu-ray discs. Even if they don't get damaged or corrupted (which they might) you can't trust them.
The affordable blu-ray discs store 25 gigs so in order to back up a typical hard drive you need about 10 of them. When your data has changed, you need to have some kind of system in place where you can back up only the changed data to a new disc. To save money, you will want to do this about once for every 20 gigs or so of new data.
- it's expensive.
- it takes a long time.
- it's not automatic
- Because of the above, you will likely not do it as often as you should.
This is an unreliable backup method, so again, you can't trust blu-ray.
To solve your problem, as others have mentioned - you don't need a NAS. Get 2 (or more, but not less) external drives and plug them in as needed. One external drive is equivalent to 20-80 re-writable blu-ray disks, and you can set up an automatic backup that can run in your sleep or while you are at work.
Back up daily and switch them weekly, or monthly. Keep one of the drives at a friend's house and you have enterprise quality storage.