Which large groups of blacks were the first to agitate against segregation? why?

The correct answer for who they were is African-American soldiers who came home after world war 1.

There was a huge amount of African-American soldiers in world war 1 who volunteered to join the war effort. They got organized into segregated units and were dispatched to the worst places in Europe because they wanted to fight for freedom and equality and against the German aggression.

When it comes to why, that is because they were still treated badly when they came back home. They defended democracy and fought against tyranny, only to come back home and be greeted in the same tyrannical manner, instead of being praised. They still suffered from heavy racism and segregation and wanted their rights.