As wiki notes, HoD is criticised in postcolonial studies, but since it is greater than them, who cares? I suppose I do at least a bit, since I noted it. Also, the complaint is at least partly illuminating: but the answer is that the novel isn't really about Africa, except in the imagination. Does that make sense? I know what I mean, even if I can't explain it very well. Anchored in the Thames, a primordial river, Marlow looks back to the lights of London and sees the darkness there; the novel is about the opposite of that, which could be the Congo or it could be the Amazon and doubtless other rivers too. Since Apocalypse Now, it could be 'Nam.
The story of travel to and arrival in Africa, and the setup of the - I assume - Belgian Congo, and the travels in the river boat are of some mild historical interest. The descriptions of the fighting, and reaction to it, are in stark contrast to the usual "heroic" type accounts one hears; I'm unsure how novel it was on Conrad's day1.
The classic beautifully setup piece is towards the end, when Marlow relates how he thought he was dying, and realised he might well die without having anything to say; and contrasts himself with Kurtz who, despite the terrible though largely unnamed things he may have done, did at least have something to say. Arguably that resonates less in today's world where every fool thinks they have something to say and generally does; but it isn't addressed to the fools.
HoD has in some ways the good fortune to be addressed to a huge topic / thought / meme: the Heart of Darkness of the title. You can project a huge swathe of ideas onto that. Nowadays, with Apocalypse Now, it gains an extra resonance. I contrast it with Gatsby, which is essentially trivial.
An output of progress
Special bonus report: AOOP. A short story, next to HoD in my collected Conrad [text]. In some ways a mirror-image of HoD: a tale of two white men left to run a minor remote company output who, unlike Kurtz, have no particular character, no energy, and little resilience in the face of the unknown.
Like Kurtz, they are trying to collect ivory. Unlike him, when faced with the unknown, they have little idea of what to do and settle for doing little. Towards the end of the time before the company relief is expected, a group of non-local blacks come through, carrying (primitive) guns, and appear to leave after looking around. The local company black brokers a deal, and in the dark of night the strangers return and take the stations worker-blacks, in exchange for some magnificent tusks. The two whites are outraged by their people being taken into slavery - though the same workers had been little but slaves for them - but are too ineffectual to do anything. Weakly wracked by guilt, they fight when the relief boat is late and food grows short; one shoots the other by accident; and the last hangs himself when the relief boat finally arrives, in thick fog.
Just like in HoD the whites, though pathetically vulnerable, are not menaced by the blacks, who fear the unknown consequences of harming them. Without realising it they are dependent on the local villagers, and their decline largely stems from the village head deciding not to feed them, after some of his people were taken in the course of the slaving exchange.
There's no particular moral, as such, other than a reminder of how dependent we all our on the support framework we so take for granted.
Beasts
John Crowley in Beasts puts forward the reverse: Meric, returning briefly to the Candy Mountain, finds the leos have relieved him of the burden of speech.
Another view of London
Composed upon Westminster Bridge, September 3, 1802.