Another way of doing this is to make your selection of the area you want to paint, then create a new layer (shift+ctrl+n).  Your new layer should be now active with your selection still in place.  Now when you paint , your strokes are going onto the new layer not your original layer.  Now you can adjust the transparency slider of the new layer without affecting the original image underneath.  You can now perform any function on the new layer to get the effect you want (blur, erase, levels etc).

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