Simply not true. DeKalb is as you say. Fulton county stretches North and South and has most of its population in the southern (Atlanta proper) part. North Fulton is solidly Republican (note I don't say conservative); however, their vote is always overwhelmed by the black entitlement mentality democrats who live and breed in the lower half.
Gwinnett is overrun by hispanic "immigrants" and liberal yankees who having sh!t where they ate up north, fled south to a better life only to take their big government statist attitudes with them and promptly begin to screw things up here. Still Gwinnett mostly goes Republican.
Cobb is contaminated by the aforementioned yankees, but still is mostly Republican. Clayton is changing not so many immigrants, but the Atlanta side of it does have a lot of blacks who've moved into middle class developments, but retain their hard core racist ideas.
Clayton county OTOH is totally third-world already: 83% for Bonzo in 2008 and this was a county that had once given Reagan nearly 75% (1984).
Henry County, Douglas County, Newton County: going downhill now, and will likely pick up speed; they will be the new DeKalbs & Claytons within a decade, probably less.
But areas on the outer northern ring of the Atlanta metro area look for now like excellent places for families and businesses to locate.