This article details the effects of the different normal map blend modes available in the Blend (Normal) node.
The Normal Detail blend mode preserves the flat areas in the background normal map, while adding the normal detail from the foreground map and orienting it along the surfaces of the background.
The background input (left), foreground input (middle), and output (right) of a Normal Detail blend at 50% opacity.
Normal Inverse Detail
The Normal Inverse Detail blend mode works identically to Normal Detail, but instead makes the background input’s normals follow the surface of the foreground.
The output of a Normal Inverse Detail blend using the above inputs at 50% opacity.
With the Normal Combine blend mode, the foreground normal map is placed directly on top of the background. At full opacity, it replaces the background completely.
The output of a Normal Combine blend using the above inputs at 50% opacity.