しかも 'moreover / and what's more' opens the dramatic reveal. あるのは '(the one) that exists' is the cleft-like nominalizer introducing the focus. 汚れじゃなく 'not a stain (but)' uses the contrastive negation pattern X ではなく Y 'not X but Y.' 血判 '(a) blood seal / blood signature' is a historical sinitic compound for the samurai/yakuza practice of sealing an agreement with one's own blood — an extreme commitment. The stuttering け、 marks shock.