The very concept of the titular Blob Monster: You can't reason with it, scare it off, or kill it. It's just an unstoppable mass of muck that exists only to feed and grow in an endless cycle. You can't even hide from it, as it can seep through even the slightest crack or hole to reach you.
If you get caught by the Blob, your death is going to be excruciatingly painful, as it slowly dissolves you alive.
The theater scene, where the Blob devours the helpless projectionist and then starts oozing out of the filming slots to begin washing down over the theater seats is notorious.
There are a couple of verbal acknowledgments of what the Blob does to its victims; but our first visual indicator comes when Doc Hallen checks on the old man, whose entire forearm is coated in Blob rather like an opera glove. But then the Blob begins to distort, and you realize that his arm is no longer there.
