We aren't too far from you and it's definitely not a law around here! I've heard of places doing this before, but it doesn't make it right. A blob is a blob. It may mean nothing to some vets. Owners use tats as a form of identification, so it could be interpreted as that. I'll have to ask Ellie's vet what she would do if she saw something like this. If a dog was rehomed (stray or whatever) and it came in to be spayed, I somehow doubt that a blob is going to stop a vet from checking. B/c if it was another kind of ID mark, the dog is at risk of disease. And to put the actual word on the dog..it can fade.
I suppose they did inform you though, so probably not their fault. But I wonder just how hidden in the consent it was. I don't have tats myself b/c of my own feeling about them and I'd be way less than happy to see one on my baby.