Pest/Disease ID

Can anybody help me ID this problem with my spanish mint plant?
There are lots of tiny blobs, almost like blisters or maybe egg sacks. I can't find any bugs but they might be hiding from me.
The plant arrived with a couple of leaves like this, which i cut off, but i came back about a week later and the problem had spread.
I have attached a picture but it is not a great attempt. It is the best i can do, sorry.

Any ideas?