Take a look at these examples:
Creating a Gaussian Blur with VB.NET[
^]
Gausian and Alpha Blurring[
^]
Both uses 2 loops, which changes pixels from 0 to Width/Height of Image:
For PixelY = 0 To bmp.Width - 1
For PixelX = 0 To bmp.Height - 1
All what you need to do is to blur pixels occupied by number plate ;)
|----------------------------| <--- image
| |
| |
| |--------| |
| |--------| <------------------ number plate
| |
| |
|----------------------------|