The blobs (called Monads in the story) are:
enormous, bacteria-like creatures eating the world away from its corners. On the map, they could be represented by black semi-shapeless silhouettes, a series of connected black circles, etc. any kind of simplified and "symbolic" representation that there is something bad there.
After a few options were presented, and the decision was made this one had the desired effect, so it is what we went with.

Thanks for the Kudos.

-Rob A>