Is it right for Robert Downey Jr to "black up"?

Downey Jr is a racist bastard, he deserves to burn on the hell fire.

I'm afraid that you have misinterpreted the movie's synopsis. Kirk Lazarus is supposed to be a white Australian method actor who immerses himself so entirely in his role that he has undergone a "skin dyeing" procedure in order to be able to play an African American. So it's entirely appropriate and no different to Eddie Murphy playing an elderly Jewish guy in Coming to America or the Wayans brothers in White Chicks.

The point is an actor often has no sense of self. That is why they escape to playing roles in the first place, because they have a damaged ego. It makes no difference what color he paints on top of the damage. The other point of it is to laugh at the face of people, who get offended by a black face joke, and it is making a statement about the PC-culture by doing so. Other examples of this were the panda bit and the kid flying in the air, obviously directed at the easily outraged crowd, nowadays known as the SJW:s.

The character Kirk Lazarus is a blonde and blue-eyed Australian. This Australian is playing an African American character. Kirk Lazarus is taking himself immensely serious doing so. RDJ is the one portraying Kirk in a comical way. This is how it works. RDJ isn't representing black people. He is representing method actors that consider themselves being capable of everything.

has a thing for the dude portraying the dude playin' the dude, disguised as another dude

He wasn't playing a black man. He was playing an idiot white actor playing a black man. Thus, he was actually making fun of idiot white actors who think it's acceptable to deprive black actors of jobs that should be going to them.

It's hilarious, and in its own way kind of PC, since the joke is the attitudes of certain white people.

I don't think it's really a matter of "right or wrong". That's like asking is alright for a man to portray a woman, or a perfectly able-bodied person portraying a wheelchair-bound person. It's all acting. The reason he was able to pull this off had nothing to do with the "theory" that everyone in the audience would understand he as a blue-eyed, blond-haired Australian portraying his idea of black person and that everyone should just relax because it's a comedy. The reason this worked is because he did this role in black make-up but not in BLACK FACE. He's wasnt talking about being down on the plantation or wanting fried chicken & watermelon or constantly off-screen taking a nap. It worked because even though some of his mannerisms were stereo-typical, they were very relate-able and in no way offensive which is why you ask a black person about TT, they will either say Lincoln was their favorite characters or hit you with one of his quotes.

Grow up. It's fn hilarious.

To me, he was so good at playing an Australian,..an (accomplished:)) foreign White Actor playing a (overly stereotyped) Black Dude,..he should've won the Oscar, easy.

He only messed up once when he said "Jheri Curl", they didn't have those in Nam in the 60s-early 70s, those came out in the late 70s-early 80s:):)

I saw the (satirical) take of the joke actually being on the Actors, not Black People, not at all (as a Black Man). And even if I did, he was hilarious. So was Cruise.

Just as they weren't demeaning the Mentally Challenged (to me), they were demeaning the actors that play them.

Maybe I liked the movie too much; which I admit it's a fav but I didn't see the claims others saw.