Questioner
Would Allomancy affect Shardplate or Shardblades?
Brandon Sanderson
It cannot affected Shardblades. Well, "cannot" is a strong word. Things with innate investiture are much more difficult to affect with any of the magics at all. Which is why it's very hard, for instance-- Szeth is not able to bind people, or Lash people wearing Shardplate to the ceiling. In the same Allomancy would not be able to Push on it without some help. Duralumin and a really strong [Steel]Push could probably do it.
Questioner
I was just wondering if it's actually metal.
Brandon Sanderson
Oh yes. It is metal-ish... it is metal enough for Allomancy to work on it.