Lightsabers and blasters both use plasma beams.
The recent canon novel Lords of the Sith consistently refers to blaster bolts as "beams of plasma". For example, on page 13 it says:
[Vader] fired and his blasters sent twin beams of plasma into the aft shields.
Similarly, both the canon and Legends Wookieepedia articles cite sources which say that lightsabers use a plasma beam (canonically, the beam is formed by a kyber crystal focusing energy).
Canonically, cortosis is actually effective against blaster bolts as well, and works against blasters and lightsabers by acting as a conductor of the plasma energy.
In-universe, cortosis is touted specifically as "lightsaber resistant" because a lightsaber is more difficult to protect against. A blaster emits an impulse of energy, but a lightsaber can apply a more sustained discharge of energy which must be dissipated.
Out-of-universe, cortosis is touted as "lightsaber resistant" because it has historically been depicted with different and contradictory effects.