I actually thought that Lucario shouldn't be dark type at all, and the steel and fighting type was actually the correct type.
But after reading CourageHound's post:
Jellicent and its pre-evolution were most likely inspired by the Nurarihyon, a jellyfish youkai that is either shown as a demonized jellyfish or as a jellyfish-like man who wears refined, sometimes regal, clothing. It is also possible that they was inspired by the Kurage no Hinotama, a sort of jellyfish-like will-o'-the-wisp.
Lucario appears to be based on the Egyptian god of funerals, Anubis, who has the head of a jackal. Anubis's role in Egyptian mythology was to judge the hearts of deceased souls based on their actions during life.
Seems kinda dark type-ish.
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!