How many black presidents was there even before George Washington?

No. All sorts of urban legends exist to the effect that there were, the most common confusing a white politician from Maryland named John Hanson who served as the ninth president of the Continental Congress and the third under the Articles of Confederation with an African-American of the same name who lived much later and was president of the American Colonization Society and a Liberian senator. But the office of President of the Continental Congress was a ceremonial position utterly unrelated to that of the presidency of the United States, an office which did not exist until the adoption of the Constitution in 1789.