I would probably believe something signed with my own PGP key enough to thoroughly investigate it. If I found something packaged with a blood sample I probably would not be willing to pay to check the sample, because I'm a minor and the costs of testing a DNA sample are something like a year of income for me. Since I wouldn't verify the sample I would probably take the message about as seriously as I'd take anything else in my own handwriting with my signature, which is to say I'd put in several hours of effort but not much more unless I found confirming evidence. If I found a video of myself saying things, accompanied by a PGP sig and a PGP-signed transcript, which did not include any subtle signals of coercion that I could have potentially sent, I'd be very confident.

Have you ever read your own diary from fifty years previously? :)