This was more true in the era of overt white supremacy, when majority-white electorates would refuse to vote for black candidates, and thus black candidates cou

This was more true in the era of overt white supremacy, when majority-white electorates would refuse to vote for black candidates, and thus black candidates could only win elections in strongly majority-black districts. This has changed from the 1990s, which black candidates started winning even in majority-white districts. However, this also led political marginalization as well. To tilt a purple state Republican, you gerrymander districts so that you have more slightly-Republican districts than strongly-Democratic one. Concentrating black Democratic voters in one district is a strategy both to guarantee the presence of a few black representatives as well as an overall Republican majority.