The bracha after megillah is made after its rolled up ?? u sure ? I recall the opposite.
there is a sicha from the Rebbe Purim 1956 where the Rebbe goes into a long arichus which way to do it, lmaskana the Rebbe says to make the bracha before rolling it up.
If you look on videos you see the Rebbe starts to roll up before the bracha then stops and continues after the bracha