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.
And blemish is an irregularity on the surface of a gemstone, which means is easy to remove during gem cutting.
Some of the blemish occurs during the gem movement. Gemstone blemish can also happen after cutting if there is a poor handling. It is advisable to pack well to avoid scratches which can lead to blemish, so pack well and differently.
But one uniqueness about inclusion is, it can help identify the gemstone if is natural or synthetic.