the legitimacy of marriage.
i glanced over a few legal things: so here goes (read my disclaimer in my footer)
to avoid the whole state/church licence stuff there is a 'banns for marriage' thing which is about publicly notifying your intent to marriage, so that anyone who can see any reason why the parties involved should not be married can be notified.
so before doing a blockchain marriage certificate you should 3 weeks prior, do a public announcement of intent to marry. .. and whats more internationally public than the blockchain
the licence thing is normally to shorten this delay.
i have not yet researched the ceremony itself but in general as long as its publicly shown that both parties have committed themselves to each other, then a marriage is valid. (please check local law of your area about requirements of ministers/registrars)
From what you have glanced over: have you seen anything about the notification requirements? Is there a minimum number of people that generally would see any such notification (like newspaper circulation)?