Normal maps are usually saved in one of the two most common formats: OpenGL or DirectX. Blender uses OpenGL specification. Trying to use normal maps saved in another format will result in the wrong visual effect. To convert normal maps to the desired format the “OpenGL – DirectX Normal Map Converter” node group from the BIS material library can be used.
The samples of use:
To convert the normal map format from one to another you need just to invert the node group “Factor” value.