He was going to get killed and Luffy saved his ass by shielding him with his DF power, which dropped Zoro's jaw. Zoro agreed to join Luffy after seeing Luffy's DF power.
And again; why does it matter? Did Luffy continue to blackmail Zoro in the entire journey? Zoro could leave the crew whenever he wanted so this is a stupid argument.
Zoro willingly accepted to be Luffy's underling and he could leave whenever he wanted. Not only this he took one step further and truly become Luffy's underling when he begged his enemy Kuma and begged his enemy Mihawk for Luffy's sake. The guy can't stop begging to his enemies for Luffy's sake. He must be the best underling.
He said Luffy is stronger because he is the captain. Both in fishman island and against Pica.