It was inside a meteor that crashed in the Simpson's backyard. Homer started poking it with a stick and, thinking it was a "space marshmallow", tried to eat it. Although the Blob attempted to get out of Homer's mouth, he ended up swallowing it. During the night Homer then started to become a blob himself and began devouring everything and everyone in sight.
Behind the Laughter
The Blob was a monster in the film of the same name.