Whilst no particular fan of the UN personally, the blue beret and/or mandate does offer an element (thin veneer?) of legality to the act of pitching up in someone's garden, possibly uninvited, to sort his problems out. It's easier to turn a nation against a single "invader" than it might be to do so against "the world".
but the moral authority to act exists because of the circumstances of the particular mission and not via political decree.
The morality or otherwise of a mission depends entirely on where you stand politically/ethnically. The Hutus of Rwanda believed they had the moral right to massace the Tutsis in revenge for past injustices. Had they brought that viewpoint to the UN debating chambers I suspect they wouldn't have got a look in.
No mission will go ahead without political decree, no matter how moral it may be. An arse-covering politician will get as much backing as he can for his mission to maintain a legitimacy on the world stage. Running it by the UN is a handy way of doing that. Whilst they never specifically received a mandate to invade Iraq, George Bush et al didn't half spend a lot of time trying to get one...
The UN is an imperfect, monolithic, bureaucratic organisation that hoovers up money and churns out little effect in return. Notwithstanding that it does provide a global forum for nations to debate issues and provide a legitimacy to operations (humanitarian or otherwise) that might otherwise provoke further conflict where people only sought to bring peace and relief.
It's a different matter I guess if you consider yourself to hold the moral authority without discussion and are militarily powerful enough not require any legitimacy to act wherever you see fit.....