How many black people are born in Japan and are considered citizens?
there are many blacks (African-American) born in japan on the us military bases (not considered japan for citizenship) and at Japanese civilian hospitals to American couples
if an African-American and a Japanese have a child in a Japanese hospital and if the American has a working, student, permanent resident, or any other visa that states he/she is legally entitled to reside in the country the child is of American and Japanese citizenship. once the Japanese spouse enters the child's name on the Japanese family registry, the child is Japanese as far as japan is concerned. the us embassy can record the birth if both parents complete necessary paperwork but if they do not, the American side basically will have no idea if the child is to be counted/considered American, especially if the American parent does not apply for a passport for the child.
there are many africans and others that may be considered "black" --remember Black is not a nationality-- that marry Japanese but their offspring are deemed foreign by the average Japanese.
as far as tracking the numbers of "black people" that are born in japan and considered citizens, it will be a difficult task since most documentation will show the nationality of the foreign spouse.