James Ellroy's neo-noir is meant to be noir that is "dialed up to eleven" and tries to more accurately reflect the darker undercurrents of the time periods he writes about that are not reflected in the noir from the time. So you can think of his books as operating as social commentary both on the time period (e.g., the 1950s, in the case of LA Confidential), and as literary commentary on noir itself. So Ellroy's use of racism, sexism, voyeurism, and lots and lots of violence is all way more prevalent than in period noir which, even at its most exploitive, was nowhere near as ugly as Ellroy's works.

So the real question to ask is: was the 1950s LAPD as racist as Ellroy makes them out to be? Because that's the sort of thing that would not have been in 1950s noir fiction, which was largely pro-police. (I don't know the answer to that. There were certainly plenty of institutional racism in Los Angeles and the LAPD. See Mike Davis's amazing City of Quartz for a historical exploration of the racist dimensions of Los Angeles. But was it like Ellroy makes it out to be — where racism is the primary vector along which almost all LAPD activity was considered and executed? I suspect Ellroy exaggerates quite a bit, but his general point, that racism is baked into the modern American DNA, especially police, is backed up by research.)

I'm an Ellroy fan (I've read basically everything he's written, some of them several times — White Jazz and American Tabloid are my personal favorites), but you have to sign on to what he's trying to do in order to not be just appalled by it. My reading of Ellroy is that he ultimately thinks that some of these themes (violence, criminality, police brutality, voyeurism, racism, sexism, etc.) are baked deep into the American postwar experience. (No doubt earlier, too, but Ellroy concentrates on the 1940s forward.) It's deliberately meant to combat the sterile, "apple pie" views of the pre-Watergate 20th century that we still see quite common today. (This is the "Great" period that many in the white upper and middle class would like to harken back to.) There's a sensibility, especially by American conservatives, that basically everything prior to Nixon was relatively rosy. Ellroy strongly disagrees, and his dare is something along the lines of: pick an institution, a movement, a time in recent American history, and I will show you how that under all of the glossy memories, there was an ugly, raw id behind the scenes. So common themes in his books are that the line between the police and criminals (or the FBI and the mafia) is essentially one defined entirely transactionally by power relations, not by any moral or even legal sensibility. It's a pretty bold argument! And yeah, it's fiction, 100%. And certainly exaggerated for the sake of fiction. But that's the argument. I think he's more right than wrong, even if he exaggerates quite a lot for the sake of the book. (I will say, his 1960s J. Edgar Hoover, as seen in his Underworld USA trilogy, is pretty spot-on, as someone who uses FBI files for research. As is his Howard Hughes.)

Which is to say: I wouldn't read Ellroy as a commentary on noir from the 1940s and 1950s. In fact, if you find Ellroy thrilling, you'll find noir from that time period pretty tame by comparison, in my experience. (For an excellent noir selection, see Penzler's The Black Lizard Big Book of Pulps, which has quite a wide cross-section, well beyond the "old-standbys" of Chandler and Hammett.) Ellroy is taking the ethos and some of the tropes (femme fatales, etc.) from that literature, but his writing style (especially from LA Confidential onward) becomes something entirely his own (White Jazz is perhaps the biggest break), as is the direction he takes plot and themes. My sense, to answer your question directly, is that period noir rarely had explicit racism of the sort that Ellroy invokes — but Ellroy would say that this was just a society in denial of what it was really about.