I doubt Gaussian blur is an accurate model of real-world situations.

At the end of the day if you are worried about the codes being painted over print a few out and paint over them. Then scan with a variety of scanners.

If I had to come up with some more digital tests I would guess that a few of these are more representative of real-world situations:

1. Lower contrast. For example lighten or darken the whole code. This would simulate things like scanning in low light or with glare.
2. Block out sections of the code. This will test error correction levels and simulate partial damage or pockets of extreme glare.
3. Skew the code in various ways. This simulates the perspective shift of people scanning the code from an angle.

Ideally combine them in a bunch of scenarios then try to scan with a variety of scanner implementations.