How is the 15th amendment a step beyond the 14th amendment

Following its ratification by the requisite three-fourths of the states, the 15th Amendment, granting African-American men the right to vote, is formally adopted into the U. S. Constitution. Passed by Congress the year before, the amendment reads, "the right of citizens of the United States to vote shall not be denied or abridged by the United States or by any State on account of race, color, or previous condition of servitude." One day after it was adopted, Thomas Peterson-Mundy of Perth Amboy, New Jersey, became the first African American to vote under the authority of the 15th Amendment.

(It gave black men "previous slaves" the right to vote)