The cab driver from "No Country for Old Men". He was the only character who seemed in tune with the situation, even more so than Sheriff Bell.

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Sure, there was the blind man in O Brother Where Art Thou, the old woman in Ladykillers and the janitor in Hudsucker Proxy
Not very prominent characters, it would be interesting to see a Coens film with a black lead actor.

Memorable:
Marva Munson, from The Ladykillers
Sheriff Wyner, from The Ladykillers
Othar, from The Ladykillers
The Blind Seer, from O Brother Where Art Thou
The singing gravediggers, from O Brother Where Art Thou
Memorable, but kinda annoying:
Gawain, from The Ladykillers.
Fairly low-key:
Meurice, from Blood Simple.
Tommy, from O Brother Where Art Thou
Moses, from The Hudsucker Proxy

I'm not sure, but sometimes I get the menstrual cramps REAL hard.

I have just watched Blood Simple now, for the first time. The black bartender, whose name is Meurice is very memorable, probably because
he seems to be the only sane person in the whole movie.
. The rest of the cast is great too, especially Emmet Walsh.

The cab driver in Big Lebowski. I know he wasn't too big of a character, but he was funny and very memorable.
And check out Blood Simple.

What does "what's that supposed to mean" supposed to mean?

wow, what the f is that supposed to mean?

"The Coen brothers films are usually very patriotic American movies".
Huh?
"facts are stupid things" - Ronald Reagan

The Coen brothers films are usually very patriotic american movies, so they don't really tend to create a black character as the main role as it wouldn't suit the setting.

who cares for goodness sake
and Ladykillers was good "left my wallet in el segundo"

Moses in The Hudsucker Proxy
Tommy Johnson & The Blind Seer in O Brother, Where Art Thou?
Gus Petch in Intolerable Cruelty
Marva Munson, Gawain and the Gospel Choir in The Ladykillers

This Moses guy in The Hudsucker Proxy is pretty memorable.
"facts are stupid things" - Ronald Reagan