To me I think it depends on how sentient the blob is. Like if it's sentient like a human would you feel comfortable referring to them as an it? Right now based off the not being able to hear or speak then I'd probably lean toward it being safe to call them it.
I had a kind of similar situation in my Draygon books. I had a dragon character that in their dragon form I called them "it" but the second the dragon took on a humanoid form it didn't feel right to keep referring to them as "it" because they were essentially human at that point, but my dragons also have no gender which is why in their dragon form they were "it" to begin with, so I started to use they/them. They were as sentient as the MC is and thus deserved a pronoun. Likely whenever I get around to editing it I'll always use they/them instead of it in their dragon form since even in that form they're still very sentient. At the time I wrote it I didn't know they'd take on a humanoid form (or even become important to the plot LOL) so the "it" use was fine, now not so much.
Basically if the blob acts like a human or even a blob with real feelings who just can't speak or hear, give them a pronoun. If they don't, then you're probably safe with it.