Never. The worst case scenario from the POV of any country, and in particular of any country that isn't one of the big five, is not one of the big five would be that such a force exists, doesn't need to be borrowed from national armies, is obviously under the control of the Security Council, and by some stroke of luck there is an issue where the big five are in agreement.
So that an international foreign legion answerable only to the Security Council gets fielded to, say, Serbia at the time of the Kosovo war.
That would be obviously bad news for Serbia (that's the country that isn't one of the big five, in this case), but on top of that none of the big five would ever be entirely happy with this, because the legion has to be commanded by committee, a committee in which you don't count just on more or less pliant allies (like in the NATO/OTAN). You'd have to convince the Russians and the Chinese about every military decision.
Simultaneously, the legion isn't tied to any one country, unlike the borrowed UN forces of OTL.
A nightmare for any sovereign country. So, since nobody wants that, it does not exist and won't anytime soon.