Significance of the title: Heart of Darkness

"I believe the word was, at that time, used synonymously with the word slave."
Not in Conrad's time - slavery had been abolished within the British Empire about 100 years before Conrad was writing, and in the USA about 50 years before.
Conrad did write another book, titled "The Nigger of the Narcissus," in which, if I recall correctly, the term has no perjorative connotations. I think the word is simply what sailors, at the time, called black people. The more refined "negro" might have been used by middle and educated classes.

Until political correctness, the word was generally a denigrating term for black people, but not always. So, in my childhood, (50 years ago,) we had a nursery rhyme "Ten little nigger boys" which had no derogatory significance whatever. On the other hand, if I'd called a coloured person a nigger (not that I ever saw any, except once) my mother would have been cross. Not as cross, however, as she was when I, not knowing the significance, used the word "wog." So, to us, "negro" was the polite term, "nigger" less polite but could be used in context, "wog" abusive and totally unacceptable.

Of course in other countries and cultures, "nigger" was always used abusively.

In respect to the "synonymous with slave" idea, it is perhaps interesting that "Uncle Remus," a (fictional but generally reckoned to be "authentic" as to speech) ex-slave, uses the term nigger as a contemptuous word for fancily dressed town negros, as opposed to the rural plantation negros.

In "Heart of Darkness," if Conrad has contempt for anyone it is the Belgians. They are not, I think, mentioned by name, but there are enough references in the book to ensure that anyone of Conrad's time would have known who he was talking about. The natives are victims of an empire that is run without the "idea."

Words do carry meanings - two of them to be exact. The first one is its dictionary definition (denotation) and the second is the emotional associations we attach to a word (connotation). The latter is subject to change - sometimes radically - as time passes. This emotional component is what makes language sometimes so powerful and often inflamatory. (Think of the words "gay" and "queer" - which have very different connotations today than 100 years ago).

If I remember history correctly, it was after the Civil Rights movement of the 60's that the N-word became a socially uncacceptable term - but that prior to that time, it was a synonym for "black."

We cannot expect writers writing prior to a time of connotative change to anticipate that change and write politically correct prose for a standard that does not yet exist. When I teach Huckleberry Finn and Heart of Darkness, I ask students to pretend the offensive word isn't present and then ask them to examine the author's portrayal of the black characters, as well as the central character's attitude towards the black characters and the portrayal of the white characters as well. The real test of a "racial" book is the underlying theme of racism, more than language that is (today) clearly "racist." If Conrad and Twain are racist because of the use of the N-word in their narratives, then they have done a bad job of expressing their hateful ideology because both books have white narrators who are openly sympathetic to the black characters and openly critical of the white characters. Aside from Huck (and a couple minor characters), what white character in Huck Finn is admirable? Ditto in Heart of Darkness: aside from Marlow, what white character is there to admire? Just because derrogatory language wasn't used towards the white characters doesn't mean that they were left unscathed by the writers.

And yes - HOD is a wonderful book. Conrad is a prose virtuoso (English - the language he wrote HOD in was his 3rd language!), and his opening 12 pages or so are magnificent. The opening scene aboard the Nellie is a masterpiece of setting and the establishment of tone.

I agree with your central point regarding the connotation and dennotation of words. I think that is an absolutely fundamental characteristic of language that does not get the attention it deserves.
I do think that your post can use some clarification. What do you mean by a "racial" book? I honestly do not know what that term is referring to. Do you mean any work that portrays characters of different races but the theme of race may not be central? That would be a huge category. Or is it a book in which the author deliberately focuses on race as an important theme? That is, works by Wright,Baldwin,Twain,Walker, Malamud...

Also, I would argue for a broader definition of racism? Twain was clearly throwing punches at the racist ideology of his time. But I would argue that one can critique an ideology, a theory, a statement but also be complicit in it at the same time. There are many whites who work against racist policies and acts such as targeting people of color by the police or the economic poverty of the African-American community. But these white people, and I include myself, can still have deep unresolved feelings, judgments, fears, bias towards people of color. We have a rather superficial understanding of oppression in this society and that is why blow ups like the ones by Gibson and Michael Richards get so much attention.
The vast majority believe that if you do not say the N word and if you don't go off on some racist tirade wearing Klu Klux Kan sheets then you are not really racist. Well it just aint so. Gibson is anti-semitic regardless of what he may argue and Richards is racist. In fact I find it absurd for anybody to claim that Gibson's/Richard's explosions are just innocent outbursts of anger rather then the alarming display of prejudices usually stuffed deep down in the dark crevices of the soul.

I do not write this intending to publish a rant about the idiocy that surrounds us. Although I sure am sick of it. I simply want to point to a deeper and I believe more accurate definition of racism. At any given point in history you could say that the society is...lets say 7 on a scale of 1 to 10 on a racism gague. Now Twain perhaps was a 4 at the time of his writing and that certainly deserves praise. He used irony, characterization,plot...to take racism to task. But we ought not grant him less then his due but also not more. I believe this is crucial for teachers to understand because students need to grasp the true nature of oppression and the variety of methods used to modify it, reduce it, hide it, reform it or totally eliminate it. There are substantive differences in those different strategies and they need to be spelled out.

This may seem off topic but I remember a brilliant point made by James Carroll in a book about the Catholic Church's relationship to Jews. He describes a scene he witnessed in front of Auschwitz. A group of nuns were kneeling in prayer right next to the entrance and bowing in supplication for compassion towards all those who suffer, for the Jews who were killed, for those still alive and for those who were guilty of great acts of injustice. This all seems good but look deeper...Do these nuns believe that the Catholic God they are praying to is the One Allmighty Lord only to be embraced by those who accept Christ? If so then even though they may be asserting their humanity aren't they also guilty of the same arrogance that saw Jews as lost, as against Christ, as needing some form of conversion? And since it was Gentiles who slaughtered the Jews doesn't that cast grave questions regarding the nature of their worship and system of beliefs? Their act has to be seen in the broader context of a world in which New Testament values dominate the Western ideology and all other systems are of secondary,tertiary or lower power.
I mention this only as an example where an act that seems ostensibly humane and just can also be understood to involve complex complicity.