The {blue-texture is not a null-texture. Obviously. The null-texture is called 'null', where the {blue-texture is called '{blue'.
Faces with the null-texture are not drawn, but still are solid. You can easily create a HOM-effect when you apply the null-texture to a wall, but that doesn't create a leak.
I use the null-texture to decrease the amount of polygons in a compile. Some faces will never be seen by the player and so don't have to be compiled. In such cases I use the null-texture. But it can be handy in other cases too.