What message does Claude McKay want to send in Banana Bottom?

Festus Claudius "Claude" McKay was a Jamaican-American writer and part of a movement known as the "Harlem Renaissance", in which black writers and other artists attempted to create works that reflected the lived black experience and recover the authentic voice of black traditions which had been suppressed by European colonialism.

McKay wrote Banana Bottom in 1933 to address issues of the conflict between white and Jamaican society as seen through the viewpoint of Tabitha “Bita” Plant who has just returned to Jamaica after having spent seven years being educated in England. On the one hand, superficially, she appears to be a perfect example of assimilation, a black woman who has succeeded in fully conforming to the norms of British colonial society, in part as a result of the help of Malcolm and Priscilla Craig, a missionary couple. When she returns to Jamaica, the Craigs plan for her to marry Herald Newton Day, who is also a native Jamaican, and who is studying to become a minister. Herald, unlike Bita, has fully assimilated to white colonial culture and religion. As we come to know the characters, we begin to realize that while Herald has fully internalized European values, Bita has not, and her increasing unhappiness with Herald in emblematic of her conflicted identity. 

The main message of the novel deals with the tension between Bita's authentic self, grounded in island culture, and two forms of racism. The first form is the racism of the imperial British, and the notion that the worth of a person was dependent on how well a person assimilated to that culture. The second form of racism is that of the islanders themselves who have become complicit in a racialized hierarchy. The very color of Bita's skin limits her social and marital prospects:

If she had happened to be born a light-brown or yellow girl, she might,  ... easily get away with a man of a similar complexion — a local functionary of the law courts, ... or a manager of a business ... in the city. But she was in the black and dark-brown group and there were no prospects of her breaking into the intimate social circles of the smart light brown and yellow groups.

It is only when she returns to the fully black world of Banana Bottom, escaping from the social hierarchies of not only British society but of an upper class Jamaican society complicit with imperialist ideology, that she can live authentically and be happy. The message therefore of the book is an anti-imperialist one that emphasizes the importance for blacks of finding authentic roots in black culture rather than getting caught up in European ideologies and prejudices.