Achebe has gone to a lot of trouble to find very little evidence to back his claim. His accusation is a sliver of truth under 90% chip-on-the-shoulderism and 10% invalidly applying the social values of his day to Conrad's much earlier time.Achebe: An Image of Africa: Racism in Conrad's "Heart of Darkness" - Achebe essentially labelling Conrad a racist.
Read Things Fall Apart by Achebe himself. One of the most widely read pieces of African literature.
Or, from a similar time as Conrad, try Aphra Benn's Royal Slave also known as Oroonoko.
There is generally plenty of real present-day in-your-face racism going on. Achebe's literary talent - and vitriol - could have been far more effective if it had taken on something more current and tangible.