"Blow the man down" refers to the communication system on board the older vessels. Basically a tube that ran from up on deck to "Down" below. The skipper would first blow down the pipe for attention and then bellow out his instructions to the unfortunate subordinate below.
"Blow the man down" = "Call him up"

Now I know that some of you will think, I'm pulling your legs but if you hang around long enough, I'm sure their are a number of old salts in Mudcat who will explain this one better than I.

Ahoy!
Jim