I never understood its typing fighting/steel. I mean, why steel? Because it has something pointed in its chest? There's honestly nothing to do with steel in lucario, and also it don't even learn many steel moves afterall.
Maybe they gave it this typing to give more power to lucario, and I don't know why.
But, answering the question:
As others said above, Lucario looks like it's dark typed in it's design and learnset, but by analyzing it's behaviour it's quite improvable that it could be naturally dark.
To me, lucario should be fighting/psychic,despite of it's learnset.