As requested, a scan of my Montblanc Permanent "Blue Black"
I find my idea of waterproof differs from those frequently expressed. Many are happy if the page can be read after an encounter with water. I set the bar much higher, looking for something which is completely unchanged by its watery encounter.
I don't like black. Blue could be acceptable, but blue black is the ideal. Full strength IG ink seemed promising, but a bad experience with Diamine Registrars put me off (varnish-like coating inside the ink window of an M200 after 4 days - took ages to clean).
January '16 I tried Montblanc Permanent Blue and it did all I wanted. Ran happily in two Pelikans and an L2K, but was just a little too bright. By April I had acquired a bottle of Permanent Black for mixing. A little experimenting and it seemed the addition of just 5% black made all the difference needed. In July '16 this mixture took up residence on a Lamy Studio and has remained ever since.
The ink is completely waterproof in all tests I have done, is (to me) an acceptable blue black and performs well with minimal maintenance. It has slightly stained the converter, there is some nib creep and it seems to need refilling before it's empty. To me these are minor irritations.
Here is a sample page.
Despite some fiddling, the image looks a bit too purple on my screen. With many BlBks being rather grey, it is warmer than most. But the water has had no effect at all.