Racial slurs aren't dark humour

I would have probably told him to get stuffed by the end of your fifth paragraph, thought judging these things in person is never as easy as in hindsight.
You were as harsh as you needed to be, don't feel bad for tossers

Actually, I never asked, but are you black? Or did he call you a nigger for some other reason?

They are, however, often used as the punch lines in dark humor.

I think by then he probably would've, but stopped when I told him to cut the bullshit.

If he did say it however, I wouldn't go yelling with pure rage or whatever. Even in worse situations where I've felt like I wanted to hit someone, I still keep myself together.

Honestly not sure what the perfect thing to do was, but now I'm hoping we can both move past it.

Nope, I'm a white skinned middle eastern. 

No idea why he thought it was appropriate, but oh well.

I do know what dark humor is, but I would not say a single word slur goes along with even the definition you give. The n word is not necessarily tragic nor a situation, it's just a racist remark. I wouldn't call any sort of racial slur "dark humor" as they aren't exactly jokes, they're just ignorance in word form.

Unless it were specifically part of the punch line.

I read the post where you mentioned this just after I replied. I can settle with agreeing on that. Subjectively, I still wouldn't consider it humorous, but I guess it would categorize as dark humor at that point.

The guy in mentioned in the OP was just doing it wrong and obtusely.

You've been watching some very low quality dark humour

I would argue that if you never hear racial slurs involved with dark humor then you would be the one watching low quality dark humor.

I haven't been following what Layton has said throughout the thread, but the OP seems to intentionally leave out the context in which he was called that. Did he give any context to what happened, or is everyone making assumptions based on his demeanor involving the whole thing?