Nothing, I would think - it might affect an area about 50-100 miles wide. A typical effective EMP needs to be at least 250 miles up.
While a 50 to 100 mile blackout would be significant depending on the location, (New York, L.A. or D.C.) for instance, I understand that a plane borne EMP would not have a big enough impact to shut the whole electric grid down.
“A typical effective EMP needs to be at least 250 miles up.”
And it would have to be really big, in the megaton range even then, because the pulse is diminished with the square of the distance.