In my experience, blowing only means they are alerted to something they perceive as a threat, but they are not sure of. It is a means to get all deer in the area on high alert. If they actually believe something to be a threat, you see the white tail.

I was hunting the edge of a clearing and the lead doe spotted me at about 20 yards. I was behind one of the those stupid umbrella blinds. She blew but she and the crew continued to munch away, all the while blowing and stamping at me. They didn't like what they saw, but they also weren't convinced I was a threat. Had the wind shifted, I am sure I would have seen white flags.

Unfortunately, it not being a doe day, I couldn't take any of them. And I feel pretty confident in saying that their blowing at me prevented a buck from following them out of the swamp into the clearing.