I guess in retrospect, it would ideally be up to the GM...
yes, the power acts like a short sword (at lower levels) but the way I have understood it from reading, but than again I do have a history of misinterpeting things... that it's a visual representation of a soulknife's mind blade, it's composed of psychic energy thus if dropped it disappears and the soulknife can reforge it the following round...
thus it could conceiviably leave no mark, or leaves a short term mark like a bruising, a red mark where it was struck, etc...
I guess for the prospect of the soulknife in the story i'm planning on that it doesn't leave a mark as it is a story...
but as for the actual character i'm considering... he/she would start off as a soulknife/rogue hybrid and eventually (if lives long enough) picking up the Soulbow Prestige Class