Well, normal maps are already normalised. The information stored inside it is merely the change in direction relative to the actual surface of the map. Using only the blue channel does not give you the actual height map, it gives you higher values for the flat parts of the surface.
Just in case you phrased this wrong and were talking about the cross product (which is not a simple multiplication) I made this example:
Here you can see that creating a heightmap from the normal map using the cross product and then using this heightmap to create the normal map does not result in the same normal map. Feel free to try this yourself, or point out mistakes I made, if you wish.