Before we go into the difference let us first look at the definition terms.
Blemish is an irregularity that is confined to the surface of a polished gemstone, while Inclusion is enclosed within the gemstone or reaches the surface from the interior.
In other words inclusion is an irregularity in the gemstone, sometimes within the gemstone or sometimes starting from the surface going to the inner part of the gemstone. Most of the time when gemstone has an inclusion, it is not easy to remove it. If you try to remove it by force you will end up reducing the weight of a gemstone.