Blob appears before Modern English. I don't think Will Shakespeare uses the word in a play, but it would have been a word familiar to some audiences at least.
It's probably from an earlier English, before the vowel shift, back then you'd need to trace spoken language, which we can't do, very little is being written down and preserved. By the time Johnson makes a dictionary, centuries later, the origins of "blob" are already lost like many other words.
