The closest match I'm aware of, but possibly not what you're looking for, is "Drop Dead" (1956) by Clifford D. Simak. The aliens aren't exactly blobs but they come close. And the humans turn into them:
"Forget us!" Parsons barked at me. "We aren't human any more. In a few
more days..." He grabbed the lantern and strode down the cages and
held the lantern high, so that we could see. "Look," he said.
There were no animals. There were just the cocoons and the little critters
and the cocoons that had split in half. I saw Kemper looking at me and
there was, of all things, compassion on his face.
"You don't want to stay," he told me. "If you do, in a day or two, a critter will come in and drop dead for you. And you'll go crazy all the way back home - wondering which one of us it was."
The entire story can be read at the Internet Archive.
Asimov's Green Patches has a human transmutation being precipitated by aliens (that luckily for humanity's individuality, don't know enough about electricity) in newborns. No blobs there, though - merely green patches of fur often replacing eyes, depending on the being, and linking the subject to the hive mind. The pioneers (all males - no "life-givers" among them) aren't faced with transmuted humans and immediately recognize the danger.
A planet named Shayol has alien "dromozoans" growing blobs out of humans, but they remain unmistakably human, if perhaps sometimes larger than they used to be.
Another similar story (not exactly SF) in which humans find something blobby that they only later recognize as having been other humans is The Voice In The Night by William Hope Hodgson (1907) but it takes place on Earth.