For Mark to identify the blind man and his father must mean that either or both were known to his readers;
No it doesn't. Firstly let's nail the disingenuous presumption that this text was written by Mark. We do not know who wrote this text and it was only in about 200CE that this gospel started to be 'attributed' to Mark, without any clear evidence that he actually wrote it. Likewise for the other gospels. All were completely unattributed in authorship for a century at least.
But back to your main point - all this indicates is that the person who wrote this text was aware of a claim or a tradition that the person the text refers was identifiable. And therefore the author quotes a name. That doesn't tell us that the claim or tradition was correct, merely that over decades of hearsay this name became embedded in the oral tradition. But so what if this was a real person and was actually present during Jesus' teaching - this tells us absolutely nothing about the veracity of the miraculous claim.
There is plenty of evidence that Daedalus and Icarus existed - rather more than this chap and arguably rather more in a contemporaneous sense than Jesus. Does their existence mean that we must accept that Daedalus manufactured wings that allowed him and his son to fly? I don't think so - the factual existence of an individual provides no evidence that a miraculous claim associated with that individual is true.
likewise for Simon of Cyrene, whose sons Mark identifies. If the miracle wasn't genuine, Mark would have been exposing himself as a fraud.
But the author of the text attributed much later to Mark would have had no way of verifying the miraculous claims. I have no reason to doubt that he believed the claims (on the basis of hearsay), but that is entirely different from actually having verifiable evidence that the veracity of the claim. And of course his readers (including the early church) have no more ability to verify the claims. So he may well be mistaken in his beliefs, but there is a difference between having a genuine belief that is actually wrong and being fraudulent which requires an individual to know that the thing they are claiming is not true.
Spud - I fully accept you have a genuine belief in the miraculous claims. I and many others here think you are wrong in that belief, but that doesn't make you fraudulent, merely mistaken.