its special attacks(along with Night Slash), are Darkness/Shadow.
There are only three Dark-type attacks, the rest are physical. The type is predominantly about dirty fighting, and that's what it was all about before the physical/special split.
And Night Slash has nothing to do with Darkness/Shadow. Its Japanese name is Blade Testing.
The Japanese name (Blade Testing) is probably a reference to the way in which some particularly unscrupulous samurai would test new swords in feudal Japan. This was by hiding in wait by a road (typically a crossroads) and waiting for an unsuspecting commoner (i.e. lower class, and so with far fewer rights than the samurai class) to pass by. He would then strike to kill, and in this way learn how well his new sword could cut.
Nothing to do with darkness/shadow, it's based on killing an innocent person to test your sword.
Fighting-types fight fair, and Dark-types fight dirty. This is why Fighting is super-effective against Dark, and why Dark-type moves aren't very effective against Fighting-types.