Hawaii had almost a million people at that time and is the home of multiple military bases. Barack Sr. was not the only black man in the State of Hawaii in 1961.
I stand corrected. The source I consulted said Obama Sr. was the only African black (as opposed to American blacks) on the Island at that time.
According to the Hawaiian Journal of History there were almost 5,000 blacks in Hawaii in 1960 nearly all of them being from the US and connected to the US military
1960: Blacks - 4,943 Total population - 632,772