If I call a Black person "nigger", how bad is this today? If a Black calls another Black with this word, is it wrong?
I remember I was with an African-American friend in line to get into the movie hall and I said it(the color) out a little too loud and the whole area went silent with everyone trying to avoid eye contact. It was pretty embarrassing. Don't even want to think what would have happened if I had used the n word instead. :(
Besides here is the definition from Oxford online dict:
offensive. a contemptuous term for a black person.
That should suffice to ans your first half of the question.
And for the second half, it totally depends what mood or what group is it used in. Among friends, it doesn't matter. You will see African-Americans use it in a joking manner all the time. Also, you can hear a lot of rappers using it too. Nor will it matter if an old African-American uses it. ;) But these are examples from the States as from the this wiki entry on African-Americans.
Also, check out the OED definitions for the word Nigger, which also accounts for its usage/meanings from a wider global perspective.
An interesting note on how bad the word "nigger" has become: In the 1970's it was okay to say "nigger" on television. Sanford & Son said that word in a handful of episodes (usually said by Fred Sanford). But even White People used that word in the 1970's. Chevy Chase famously said it on Saturday Night Live when speaking with Richard Prior in a sketch.
It was obviously a pejorative at the time, but it seemed to become more taboo as the 1980's and 1990's progressed. Leading it to become a word that couldn't be used on TV.
Bad. But as with any language evolution, it's a moving target as to how bad or who can use it.
40 years ago, I was living in Brooklyn, NY, and a new Paster moved into the Baptist Church on the corner of my street. The daughter and I were having a casual conversation about the neighborhood and she asked if there were many n-word in this part of the city. Even as a teen, I was sensitive to the word, and told her that the word was a pejorative, a white person just doesn't use it. She said "that's how they refer to themselves down south." I told her things were different here.
Skip ahead to today. A US-based public radio show, This American Life, had a segment on schools and school bussing (sending kids to different schools to balance out the demographics). A black teenage girl was interviewed and she said "the b-word" instead of bitch, yet she said n***er without hesitation. There's a vocal group of black people telling their children and others that using this word is inappropriate, it should be retired from the language. Others feel using it freely takes away the taboo, and 'disarms' the word. Somewhat similar to how gay people have taken the negative meaning away from the word 'queer', even to the point where a cable tv show "Queer eye for the straight guy" was on Bravo for a number of years.