This prompted some study for me yesterday. My original answer had sprung from Matthew 11:2-6, where John the Baptist is in custody and knows he is to be killed. He is having some internal doubts, and asks his disciples to go to Jesus to verify that, in fact, Jesus is who he thinks he is. Perhaps it's just John facing death and wanting to be sure, or perhaps he's heard stories like that mentioned in Matt 11:18-19; Jesus just does not match expectations. Either way, Jesus sends a reply as follows:
Jesus replied, "Go back and report to John what you hear and see: The blind receive sight, the lame walk, those who have leprosy are cured, the deaf hear, the dead are raised, and the good news is preached to the poor. Blessed is the man who does not fall away on account of me."
Both Jesus and John knew the scriptures well. This message ties to the prophesy of Isaiah Ch. 35:1-6 and 61:1. In essense, he's pointing to his miracles as proof of his Messiahship. It is not a message that would be interpreted by another other than a scholar, one who knew the scripture well, so John could be told without Herod or his guards becoming suspicious.
So, why did Jesus want to keep his true identity secret for so long? Simply, he was not ready to reveal that he was the coming Messiah to the world. He knew there would be a price to pay for making that assertion publicly; when he finally entered Jerusalem in Mark 11, he was prepared for the consequences. He was not ready yet, and so for those events that would be a signal he asked that they be kept quiet. He was not trying to keep these things from the crowd, but from the teachers of the law that might move against him prematurely.
What I didn't know was how this idea stacked up against other miracles in Mark. To check it, I looked at the occurrences of healing miracles in Mark that happened before he went public. There are five instances where Jesus commands silence; all fall in the category of the prophecy above (blind, lame, leprosy, deaf, and dead). There are 11 instances of healing that did not have the command written. Nine of those are for things not prophesied against (fever, demon possession, sickness, bleeding), but two DID fall into the prophesied category.
The first of those two instances was where Jesus healed a paralytic in Mk 1:40-45. However, he healed this paralytic IN FRONT of teachers of the law; why command silence when those you wanted to be silent from already knew? The second was where Jesus healed a blind man in Mk 10 46-52. In this case, Jesus was on his way to Jerusalem; there would have been little need to silence the man since he would soon reveal himself anyway.