The RGB channels are X, Y, Z
Red stores the left-right tilt, green stores the up-down tilt, and blue stores how much the surface faces the viewer. Each is mapped from -1..1 into 0..255, so the middle (128) means no tilt.
Why blue dominates
For any surface you can see, the normal points mostly toward the camera, so the Z (blue) value is near its maximum of 255 almost everywhere. That full blue channel across the whole image is what gives normal maps their blue-purple look.
Reading red and green
Where the surface tilts, red and green shift away from 128. A sharp edge that faces left encodes as darker red, one facing up as brighter green. That is why edges appear as red or green lines on an otherwise flat blue field.
The flat-colour value
A perfectly flat surface is (128, 128, 255) - magenta-blue. If a normal map is that colour everywhere, it contains no detail and will not change the shading at all.
- 128, 128, 255 → flat
- More red → surface tilts in one X direction
- More green → surface tilts in one Y direction
- Less blue → surface tilts away from the viewer