Why is Obama being billed as the first "black" president when others of black heritage have been elected b4?

Not only did Presidents Thomas Jefferson, Andrew Jackson, Abraham Lincoln, Warren Harding and Calvin Coolidge all have black forbearers, but even in more recient history, Dwight Eisenhower, the 34th president, also had black ancestors. His mother, Ida Elizabeth Stover Eisenhower, was half black. Obama's father was black and his mother was white. Does this make him more black. How black do you have to be to be a black President? Why does this matter so much now, when we have already had six Presidents of part black heritage? Why is so much bearing put into black heritage in this country when other heritages seem to have imilated into being American, dispite their origin, skin tone and physical differences??