You just got unlucky. Royal Mail don't examine everything that comes into the UK, but on this occasion they chose something of yours. I've probably bought several hundred coins from the US over the years, but I have only been caught for customs duties etc. on a couple of occasions. It's a lottery!
It also looks as if they are using a common shipping/tracking system with automatic flagging up to the receiving country once entered into the system rather than examining packets individually because it says they will notify you when it arrives in the uk. It's only possible because you have an account which you can settle via email. Phoning card details wouldn't work.
There is no evidence of the reverse pattern in the blob, so it was not there when the coin was struck. So I would say this is post mint damage - someone has added the blob, either to make it into something (a large button?) or messing around in the workshop. Subsequently it has gone back into circulation and got the wear to the face.