Luffy hits Caesar with regular haki and hurts him. Caesars says "That hurt." He is bleeding. That's still not enough damage so Luffy uses Hardening. He
says Hardening. His fists
only turn black when he uses hardening. Caesar
sees his blackened fists and reacts to this with fear because he
knows it would hurt more.
These 4 pages are right after each other. This should not be hard to understand for anyone.