On my passport's signature page there is now a blue ink spot, that appeared mysteriously (I keep it in a ziplock bag with my birth certificate, but also a few other documents. I have now seperated fresh documents written by pen as I believe that's the culprit). Since the US state department is vague about what constitutes damage, and I have read for example a small tear on a page can result in refused entry, should I be concerned about the current validity of my passport? Not only for entry but also for my general identification (like for notarization or opening a bank account. Where I live they only accept the actual passport for those things, not a photocopy)
No, it will not be a problem.
The only reason something even remotely like that would be a concern is if it appeared that it had been made in an attempt to modify your passport, or any of it's contents (including visas/stamps), in some way. Clearly that isn't the case here, so this will not cause you any issues.
That little dot on the left? I don't see how that could reasonably be called "damage" by anyone. Maybe you don't use your passport much, but I've had friends and family with some much more heavy marks, creases, and even short tears. I even know of a case where he ran out of space for the visa stamps, so they just stapled new pages into it, right through the old pages.
It's the ID page and official stamps that matter. If your picture is unidentifiable, they can't scan it clearly or even move it through the scanner, your signature is blotched out, etc; these are issues. It is an identifying document, not a work of art. The agent that sees it is evaluating his confidence in its legitimacy and your legal status based on that. He doesn't really care what it looks like as long as it does what it is supposed to do.
The US State Department - Bureau of Consular Affairs covers this exact issue. They give details that stress the data page and photo, the cover, and "injuries" like missing pages and water damage. They mention "significant tears", which presumably means small tears are ignored, except probably not for the cover and data page. They say:
- My passport has been damaged. Can I continue to use this passport?
If your passport has been significantly damaged, especially the book cover or the page displaying your personal data and photo, you will need to apply for a new passport. Damage that might require you to replace your passport includes water damage, a significant tear, unofficial markings on the data page, missing visa pages (torn out), a hole punch, or other injuries.
Normal "wear and tear" of a U.S. passport is expected and does not count as "damage." Normal wear includes the bend of a passport after being carried in your back pocket or fanning of the visa pages after extensive opening and closing.
That mark is very small and it is not on the data page. There is no reasonable interpretation of the text above to make that invalidate your passport. BUT, this maybe could have been worse. You are right to take measures to prevent future marks and damage.
It's relatively easy to find "denied entry" stories based on a "damaged passport", but among the first few I found that actually describe the damage, they all run afoul of the terms in the quote above:
- Data page was delaminating
- Visa pages ripped out
- Water damage "at the bottom of the pages" presumably all pages.
- Tears on the data page
Replacing your passport can be a hassle, and your example damage is so small. I don't just doubt there would be issues. I would expect there to be none.