Parker Quink ‘Blue Black’ after colour-shift to teal

This is a photograph that shows the teal colour to which the Parker Quink ‘Blue Black’ that I got in a pack of cartridges that I bought here in the UK, in October of 2020, eventually turns.

I took this photograph on 2023-05-04.

The writing below the lines written with my 1979 Parker 25 on 2021-09-20 is actually the same ink.
But I wrote it from my 1994 Parker Vector ‘M’, on 2020-11-29.

The ink goes down on the page as a dark ‘navy blue’ colour, especially when written from the Parker Vector that I used for the writing in lower portion of the photo.

The paper is a delivery note that came with a different order, so is presumably ‘laser printer’ paper.

But I have found that this ink shifts to this, teal, colour on all the papers on which I have used it.
Including the ‘ivory’-coloured Clairefontaine paper that can be found inside the Rhodia Webnotebooks that I use for journalling.
Which are kept closed, inside a closed drawer.

i.e. this colour-shift is not an artefact of dyestuff degradation by incident light.
I think that it might be caused by a reaction between the dyestuffs and a preservative that is in the paper.