Why do Black people belong to the Democratic Party if it's the party of slavery and Jefferson Davis?

Pick up a history book. The Democratic Party is the party of Jefferson Davis and slavery while the Republican party is the party of Lincoln and Ulysses S. Grant, the abolitionist party, so why do so many African Americans support slavery and confederate values by belonging to the Democratic party? I don't understand this. Why don't you join the party that opposes slavery instead?