Actually I think thats a trend nationwide and throughout most of Western civiilization. The reasons behind it are varied and no concensus could probably be reached.
My own theories:
In African-American communities people have larger extended families consisting of not only blood relations, but distant cousins, neighbors, etc. This leads to a larger support network for a person and consequently more chances for a person to talk about or vent in some way the feelings, emotions, that are causing the suicidal thoughts.
Also I've been told that in the African American community, suicide is viewed more as a form of weakness and would bring "shame" o the surviving family members.
And remember what country has the highest rates of suicide. Sweden.