The patch is small, about a fingernail in size.
Elsewhere on the packet they used two colors of ink: a very dark blue (this) and a lighter blue. I expected it to be some kind of alignment pattern, but only one of the inks seems to have been used here, so that's not it...
What is it for, why would they bother printing it? An ink+paper resolution/absorption test?
It kind of resembles the patterns used on digital paper, but your description doesn't suggest that it would be used that way.
I don't recognize it as any common sort of matrix code (2D bar code).
Reminds me of cobblestones.
That's interesting! I hadn't heard of Anoto paper before... It can't be that, but interesting tech nonetheless.