Somehow I did it again and managed to buy too many Royal Blue inks. It's far from my favourite blue hue but nevertheless useful in broad nibs. In any case, it gave me the opportunity to compare them. The first picture is of what I wrote on a scrap of Lalo 100g paper. It's not a great picture but you can probably see that there's not much difference between Montblanc, Pelikan 4001 and Faber-Castell Königsblau. Pilot Blue, which was added for comparison, is similar but yet slightly more blue (apologies for the tautology but I can't put it otherwise). Also note that the unlabelled cotton swab at the bottom is Pilot Blue.
Then I folded the paper, so that only the right part remained visible and left it exposed to the spring sun for a month. The second photograph shows the deterioration of all inks but especially of Montblanc Royal Blue and, surprisingly, Pilot Blue. Pelikan and its presumed sibling Faber-Castell fared slightly better.
Finally, I put a wet finger across the paper and then blotted it with a tissue. All inks but Pilot Blue were affected.
Bottom line: everything as expected, although I thought that Pilot Blue was more UV resistant.