I'm Nigerian (born), and I moved to LA about a year ago from the Bay Area.
The African community here isn't large like the ones you may find in places like New York, Maryland, Georgia, or Texas, but it's sizable enough. Nigerians are mostly concentrated in middle class African-American suburbs, or upper middle class exurbs. The Crenshaw district, Baldwin Hills, Inglewood, Hawthorne, Carson, Lancaster, and Moreno Valley all have sizable populations of Nigerians.
Look here for cities with highest percentage of residents born in Nigeria (2000).
Top 101 cities w/ most residents born in Nigeria
Note: The number of immigrants from Africa has increased by an estimated 167% since 2000, so keep in mind how outdated these are.
Ethiopians make up the large majority of the African community, centered in the Little Ethiopia district. Even though the community is fairly small compared to other immigrants groups in California, the community is one of the fastest- this is evident from the ongoing immigration from Ethiopia, Nigeria, Ghana, Kenya, plus refugees from Somalia, Burundi, D.R. Congo, and Zimbabwe.
Click the below link for more information on this subject.
African Immigration to the United States (Wikipedia)
Sub-Saharan African Population Profile
I don't know very much about the South American community, but here are some links you may find helpful:
Top 101 cities w/ most residents born in Brazil