Good question actually,
I believe you can use any pen, a Sharpie or Pilot pen is best to write on the print file page at the top for topic, location and date as such. You can also use them to mark what frames you like etc. Just do not press hard as to scar the neg inside the print file page.

I have used pencil and a pilot pen for the back of prints, usually in the border area for good luck. Same rules apply about not pressing hard as to not damage the print.

We have many prints from my grandfather from the 1880s and such that have pencil marking on them and no issues, but they had no idea how long to wash a print, many still smell like fixer but are in great shape.

BTW, I love where you are from. Wife and I visited in 2011. Amazing