[Sebastian only implied it, as if that's any better. Ciel may be free to trust that Sebastian has nothing to do with their passing, but that doesn't mean that he's free from all blame for inflicting additional mental anguish on Ciel.
Sebastian's words, however, are very clear and specific, and leaves no room to be misconstrued. He really had nothing to do with his parents' deaths.
Ciel slumps a little, barely enough to be criticized should his Aunt Francis see him, exhausted all of a sudden and very much wanting this day to be over with. He still manages a reluctant, lopsided smirk for Sebastian, a begrudging amount of admiration in his eyes. Those are extremely dirty tactics, and Ciel can't honestly say that he wouldn't have employed them himself.]