Your signature will never be unique, each time you sign there will be small variations. It’s really not that hard to produce a convincing signature if you have good penmanship, and even easier if it’s a digital scan.

I’ve owned fountain pens since I was little, and I usually use some deep blue ink that fits the “only black or blue ink” criteria but is particular enough that I am able to tell with confidence if it is or not the ink I was using, and forensic analysis could verifiably tell. The one I’ve used the most oxides into copper colors, I love it so much.

For digital documents, I use PGP. I don’t care if it’s dorky, if I think I could have the need to demonstrate that I said or was told something at a specific date, I will generate and store a signed version, or attach it to an email as appropriate. I care more about the cryptographic guarantees of digital signatures than the legal guarantees of paper signatures anyway