Nine black former "American Idol" contestants -- from various seasons -- are banding together to SUE the show ... claiming they were all FORCED off "Idol" as part of a racist (and illegal) plot to boost ratings.

TMZ has obtained a letter, submitted to the U.S. Equal Employment Opportunity Commission by NY attorney James H. Freeman -- in which he asks for permission to sue Idol and FOX on behalf of his clients ... who have all appeared on the show: FYI, Freeman is required to file with the EEOC before filing suit.

Corey Clark (Season 2), Jaered Andrews (Season 2), Donnie Williams (Season 3), Terrell Brittenum (Season 5), Derrell Brittenum (Season 5), Thomas Daniels ﻿(Season 6), Akron Watson (Season 6), Ju'Not Joyner (Season 8) and Chris Golightly (Season 9).

In his letter, Freeman claims he noticed something fishy about "Idol" when Jermaine Jones was kicked off on March 15, 2012. Producers had claimed Jones failed to inform them about multiple outstanding warrants out for his arrest.

Freeman claims he conducted an investigation into the show -- and found that producers had only publicly disqualified 9 other people from the show since it first hit the air ... and they all happened to be black.

Freeman now alleges that "Idol" has been conducting a "cruel and inhumane" scheme to exploit and humiliate black contestants for ratings.

According to the letter, Freeman believes the annual "scheme" begins during the background check stage of the show -- when producers ask wannabe contestants the question -- "Have you ever been arrested?"

Freeman claims the question is a violation of CA employment law ... and since his clients were essentially applying for "employment" with Idol, the show was FORBIDDEN from asking contestants about their arrest history.

But Freeman claims ... the ill-gotten information from the question, coupled with arrest records obtained by private investigators working for Idol, were used to humiliate the black Idol contestants on national TV ... and in the process, perpetuated "destructive stereotypes" about black people.

Freeman also claims the show made his clients appear to be "violent criminals, liars and sexual deviants" ... when they weren't.

What's more ... Freeman claims the show has "NEVER once publicly disqualified a white or non-black American Idol contestant in the history of the eleven season production."

According to Freeman, NONE of the contestants who were disqualified from the show based on their criminal backgrounds were convicted of the crimes for which they were charged at the time of their disqualification.

But Freeman adds, "Yet their personal and professional lives remain permanently and severely impaired by [the show's] continuing violations of our nation's laws."

Freeman's letter is the first step towards a lawsuit -- and we're told as soon as he gets the green light from the EEOC, he plans to file the docs in court.

We spoke to "Idol" honcho Nigel Lythgoe ... who tells us he was "shocked" by the "ridiculous" allegations.

He explained, "We treat everybody the same ... no matter the race, religion or sex."

"I think we've always had a fantastic share of talent from contestants both black and white ... I don't think I've ever seen racism at the show."

A rep for FOX declined to comment.