I did similar testing with ball-point pens. Eventually I ran into the ISO 12757-2 standard on archival ink, thinking that it would be a great idea to use such pens for my intricate drawings.
Turns out the drawings, some of which I actually sold, faded into oblivion within about a year. After slightly more careful reading of the actual standard, I learned that the drawings were supposed to be archived, i.e. kept in a box or a drawer, and not to be framed for full-time viewing pleasure.
The typical blue ink in the famous BIC ball-point pens (i.e. non-ISO 12757) turns black after some time of sunlight exposure, which seems fine.
Turns out the drawings, some of which I actually sold, faded into oblivion within about a year. After slightly more careful reading of the actual standard, I learned that the drawings were supposed to be archived, i.e. kept in a box or a drawer, and not to be framed for full-time viewing pleasure.
The typical blue ink in the famous BIC ball-point pens (i.e. non-ISO 12757) turns black after some time of sunlight exposure, which seems fine.