Yes, Franklin was quite racist in 1751.
Then, he evolved.
In 1787, he became president of a Pennsyvania abolitionist group, promoting the emancipation & education of African Americans at a time when the more popular antislavery prescription was liberation followed by deportation ("colonization").
The next year, in a petition to Congress, Franklin wrote, "Mankind are all formed by the same Almighty Being, alike objects of his care, and equally designed for the enjoyment of happiness." He urged Congress to "secure the blessings of liberty to the People of the United States... without distinction of color."
Mocking southern claims that Christianity endorsed black slavery, Franklin penned a parody of that view by making the same arguments for white slavery in the voice of a fictional Muslim divan.
Like everyone, Franklin deserves to be judged on his whole life, not just on a narrow slice.
Above, you cited Walter Isaacson, Benjamin Franklin; now, please review pp. 463–467.