I've never brewed with ginger before, but I would imagine you wouldn't need too much in a 5-gallon batch, especially in a Blonde Ale, that doesn't have a lot of flavors/bitterness to compete with the ginger flavor/aroma.
I'd say no more than 1/4 - 1/2 tsp of powdered ginger, added somewhere b/w 5 minutes and flameout. Real ginger, chopped or grated, would be a larger amount.