Here's a list of all the relatively successful countries with a black majority. All of them are former or current white colonies. Total combined population is a little over a million. Did I miss any?
Bermuda
Population:
66,536 (July 2008 est.)
Ethnic groups:
black 54.8%, white 34.1%, mixed 6.4%, other races 4.3%, unspecified 0.4% (2000 census)
GDP - per capita (PPP):
$69,900 (2004 est.)
Cayman Islands
Population:
47,862
Ethnic groups:
mixed 40%, white 20%, black 20%, expatriates of various ethnic groups 20%
GDP - per capita (PPP):
$43,800 (2004 est.)
British Virgin Islands
Population:
24,041 (July 2008 est.)
Ethnic groups:
black 83.4%, white 7%, mixed 5.4%, Indian 3.4%, other 0.8% (1991 census)
GDP - per capita (PPP):
$38,500 (2004 est.)
Equatorial Guinea
Population:
616,459 (July 2008 est.)
Ethnic groups:
Fang 85.7%, Bubi 6.5%, Mdowe 3.6%, Annobon 1.6%, Bujeba 1.1%, other 1.4% (1994 census)
GDP - per capita (PPP):
$28,200 (2007 est.)
Bahamas
Population:
307,451
Ethnic groups:
black 85%, white 12%, Asian and Hispanic 3%
GDP - per capita (PPP):
$28,000 (2007 est.)