That's actually not impressive at all: that's the baseline. Virtually every Democratic statewide/federal candidate for office in Georgia in recent years (who isn't named Obama) where we've had exit polling has pulled the magic 89-90% of blacks. Even when it comes to down-ballot statewide candidates where we don't have exit polling, this appears to be true based on calculations I've done on 2010/2014 races that focus on the "path of least resistance" in terms of support and demographics. It's quite, quite consistent.
The variable from election to election in Georgia is the white vote, which can bounce around anywhere from 15-16% (in those down-ballot statewide races with black candidates) to 23-24% (for top-ticket statewide/presidential races with white candidates).
The fact that Perdue only got 7% is embarrassing, assuming, you know, that it's not just an anomaly within the margin of error of the exit poll.