The original question of this thread (Dashed=true option in biblatex gives biblatex2.sty error), was addressed by @JosephWright below, and should be kept for future reference.
As I understood better the nature of the problem, I decided to edit the original title and provide an answer to it below. I also deleted my old comments so as to clean up the thread.
I use \RequirePackage[backend=biber,sorting=nyt]{biblatex}

dashedis an options for individual styles, not thebiblatexcore: does stylesblsay it supportsdashed? (I'm not familiar with thesblstyle.) – Joseph Wright Sep 06 '14 at 19:42