For one thing, I was born in the 1950s, in Texas, where they were still lynching people from time to time -- I grew up just about 5 blocks from a KKK headquarters in a town where the cops were 3/4 KKKers. We had no blacks in our schools until the mid-1970s, the last year I was in the public school system, and then it was three -- count 'em, three -- students, and they were scared out of their minds (went down the halls with their backs to the lockers, just damn sure they were going to be jumped). Things took a loooong time to improve. So I've had my experience with the genuine article when it comes to racism. And no, "real" racism is not simply relegated to slavery and the like; racism is anything which denigrates a set of people based upon their ethnicity, whether that be Black, White, Hispanic, Asian, or what have you. Racism is that which tends to keep a person down and not allow them the same opportunities because of the color of their skin. Racism is the snide epithets which belittle someone because they are of a different ethnic background from your own. (Or, for that matter, denigrating your own for that very same reason.)
As for the blacks "these guys" (If you are still referring to HPL & others, a bit unclear from your phrasing) didn't like not being "particularly good people" -- bosh! They didn't allow themselves to know these people in most cases; they judged them strictly on the fact they were black, or black and poor. And yes, they could get quite vicious (again, I suggest looking at HPL's letters for the most overt examples, but even in his fiction it comes out if you're at all paying attention. Think of "Buck Robinson, the 'Harlem Smoke'" in "Herbert West -- Reanimator", or the description of the ethnics in "The Horror at Red Hook", "The Street", etc. Or the choice of terms he uses to describe the inhabitants of Innsmouth and their behaviors. Or the terms he even uses for the shoggoths, using what were common terms for blacks, and made even more evidently so by comparison with his own terms in his letters and essays. No, this had nothing to do with the individuals, it had to do with their "race". The fact is, when HPL came to know any of these people better, he tended to find himself liking them as people, but this constantly conflicted with his preconceived ideas of people of each ethnic background (hence his comment about his friend Loveman's being a Jew, and his constant surprise at how he didn't fit what HPL thought of Jews -- something Lovecraft's wife discusses in her memoir of HPL). Bob Howard's descriptions of blacks in many of his stories leaves no doubt he felt they were essentially subhuman, at best "fine, healthy animals". How anyone could call this less than racism, even if it was a form of racism which was completely acceptable in their day, is beyond me. (And again, on the few occasions Howard knew a black person well, he found them to be an anomaly because they didn't fit his image of blacks. It never occurred to him to question his prejudice; he simply chalked it up to this individual being "different".) Just because something is considered acceptable does not mean it is right or ethical; it simply means the people of whatever time (including our own) are still clinging to traditional ways regardless of whether they are beneficial or malignant. In this case, I'd say the evidence is in that they are quite malignant, because they disenfranchise an entire ethnic group (or set of ethnic groups) of the human race.
I am well aware of the resurgence of ignorant racism in our society today -- I work with the public every day, and I see a great deal of it. But that by no means excuses these writers for harboring views which, even then, were becoming less and less tenable as the evidence rolled in, particularly as they prided themselves on keeping au courant with genuine scientific findings (though they were also prey to falling in line with pseudoscientific nonsense at times). And frankly, defending such views on the part of a writer simply because one enjoys their work shows an inability to separate one's personal like from one's critical judgment. To note and critique such views -- or any other flaws or strengths a writer possesses -- is much more honest and, it seems to me, much more respectful of them as human beings and creative artists, than to blindly defend them from that which is blatantly obvious to so many, not only of the laity, but of the best and brightest who have looked at these works, whether they are "fans" or otherwise. The latter do not duck the unpleasant side, but seek to understand its origins in these writers and how they transmuted even these things into powerful elements in their writing; in other words, to better understand and appreciate the creative genius of these undeniably talented individuals.
Without meaning to unduly cite my own work, I suggest you look up my essay "Sources of Anxiety in Lovecraft's 'Polaris'", which can be found in the Lovecraft Annual No. 6 (pp. 113-125), where I examine several different aspects of the anxiety of the narrator (including racism in the tale) and how they reflect Lovecraft's own concerns. I think you'll find that what I say about the use of critical judgment in such readings holds true, not only for that essay, but for the vast majority of those who devote their time to these writers. You might also want to take a look at Gavin Callaghan's H. P. Lovecraft's Dark Arcadia, or Robert Waugh's The Monster in the Mirror, for some in-depth examinations of such themes.