Ask them to prove it. Point out the history of the Democrat Party. Slavery(the Republican Party was formed as the anti-slavery party) Secession , The Confederate States of America, The Ku Klux Klan, Jim Crows Laws, segregation and gun control(meant to keep guns out of the hands of newly freed black slaves) were all creations of the Democrat party.
You are quite correct, historically speaking.
However, blacks, not unreasonably, view that as ancient history. And since the 60s they believe, probably correctly, that the GOP made a conscious decision to go after the white southern vote, which blacks, rightly or wrongly, view as pandering to their enemies.
The friend of my enemy is my enemy.