"Black" dahlias need sun to set the dark pigment. They should do very well in the South. The blackest dahlias I have seen are Ebony Star and Verrone's Obsidian both as black as a black pot. A picture taken with flower in front of black pot showed little difference. One problem is that both of these dahlias have open centers. In fully double dahlias the blackest I ever saw was a miniature ball called "Pot Black". Fidalgo Blacky was nearly as dark. In larger flowers, the blacks are not nearly as black. I like to think that two of our introductions Hollyhill Black Widow and Hollyhill Black Beauty are about as black as any larger flower.