Here is a quick water test of Parker Quink Black and Diamine Indigo.
Paper: HP 32-lb laserjet paper.
Pens: very wet-writing vintage flex pens, Conklin 26P and Waterman 52. Because of the heavy ink deposit, the Diamine Indigo looks nearly black, but next the Parker Quink Black, it's subtly more blue on the page. In other pens, these inks are easily distinguished.
First scan: both inks after water immersion. I intended for the lower half to be submerged and the upper half to remain dry, but it didn't turn out that way (oops--the whole sheet fell into the sink!). The lower half was submerged for 15 minutes then rinsed under a stream of water. The upper half was submerged for some indeterminate time less than 15 minutes.
Second scan: both inks, freshly-written.
Colors: In case your monitor shows the colors differently, in real life, the top half of the submerged sheet shows an orange halo around the Quink Black and the lettering looks like an intense dark blue. There is a turquoise halo around the Diamine Indigo and the lettering is a softer but still dark blue-gray. The lower half shows a sky blue halo around the Quink Black and the lettering has turned blue. There is an indigo-violet halo around the Diamine Indigo and the lettering has turned to a dusty but dark blue-violet.
I'd feel comfortable addressing envelopes with either of these inks, but the soaked Parker Quink Black is easier to read than the Diamine Indigo.