If I recall, Black turnout in Georgia actually fell in 2012- probably due to voter ID. However, the demographic changes in Georgia aren't the kind that will be canceled out by any potentially decreased turnout- they'll make up for lowered turnout an then some.
That's assuming, as Mechaman has pointed out, that current trends will continue with no modification thanks to changes in the parties. However, I actually think the trends in Georgia are more likely to put the state in the D-column long term than those in NC and VA. That's simply because in the latter 2, the shift is more dependent on moderate white suburbanites.
I don't know about North Carolina but in Virginia the shift is equally dependent on increased diversity. People don't realize that NOVA is only about 50% white. While the white population is slightly increasing in NOVA, the minority population there is booming, account for almost all of its growth. In some of the counties and cities (some cities have their votes tallied separate than the counties they are located in), minorities are almost two thirds of the population.
Right now the shift is helped by moderate white suburbanites in the DC suburbs. But sooner or later, Democrats won't need those voters much as the voting pool will become 40-50% minority. Of course, those white moderates in NOVA aren't voting GOP any time soon anyways.
