But after reading CourageHound's post:
It makes me rethink on what I wanted to post. The point on that Lucario appears to be based on that Egyptian thingy that judges souls deceased souls really seems to make it dark. But thats just a look-alike right? So, I don't really know now. The only person that really knows is the Game Freak designers. So for now, I will just say its fighting and dark/psychic, but the colour scheme makes it looks like fighting and steel/dark.
But the naming sense, also from Bulbapedia:
Lucario may be derived from the reverse of orichalcum (a mythical metal) or oracle (referring to its supernatural abilities involving aura). It may also involve lupaster (a subspecies of the Canis aureus), Cairo, pharaoh, or lykos (Greek for wolf).
The naming sense is supposed to be a metallic type of wolf. Thats makes it steel and fighting.
However, with that aura thingy, it makes it Fighting and Psychic.
So yeah that long wall of text with sucky English and lots of grammar errors is not suitable for reading, so I will summarise.
Design wise, its Fighting and Dark. Colour wise its Fighting and Dark with a little bit of steel. Naming sense, its 100% Fighting and steel. With its special aura abilities, its Fighting and Psychic.
So, why don't they just make Lucario and Quad-type Pokemon!