Any assassin should have light armor, athletics, security, and athletics. Marksmen is commonly used but since you want only blades then don't use this.
First off, Im pretty sure there's only one skill called athletics.
Anyway, I beg to differ on your statement.
Light Armor is near useless for an efficient assassin who kills from afar.
Athletics should be a minor skill no matter what, as it is used with every character. (Every character runs at some point. The benefit of athletics isn't good enough to lose a spot on the majors list.)
Security is also completely useless when you reach level 10, as at that point you can acquire the Skeleton Key. Up until that point, however, security is arguably an okay skill to have.
That's just my perspective though.