Is that whole driving NPCs away part fun, though? Feels like following and interacting with the chaos is where the fun's at, and despite making sense conceptually, the interactions with NPCs that you mentioned would get in the way of that. I'd say that NPCs that interact with the environment and/or the blob are a safer bet - things like the blob sticking to NPCs that walk around and then you have to knock if off of them, guards that mostly open/close gates and raise/lower drawbrigdes, jesters juggling balls + the blob, mages channeling some weird ritual and the blob gets caught within the ritual circle into some move pattern and so forth.
These are also suggested with different types of blobs in mind, with slightly different behaviours like targeting interactibles if it bounces close by, avoiding humans if it 'sees' them - either way it's way too much speculation at this point - there's plenty you can explore for sure bahahahahah