The group most affected by the passage of the 15 amendment were African Americans older than 21, because they could now vote. The US Constitution in its 15 amendment declared that it was the right of all American citizens to vote. Furthermore, it specified that this right could not be denied, bases on race, color or for being in a condition of slavery previously by any State in the Union.
Even tough the 15 amendment was passed in 1870, many blacks were still afraid to vote and they actually received treats, and their houses were set on fire by hate groups. More than 100 years had to pass for African Americans to get the courage to vote.
It was not until the Voting Rights Act was passed in 1965 that most African Americans mostly in the south, were finally registered to vote