The phrase the blue drop appears in The Big Why, a novel by Michael Winter.
Here is a passage from The Big Why by Michael Winter in which the term "blue drop" appears.
"The Southern Cross loaded up and hoisted mainsail but couldnt budge. She sat out there for the night, waiting for a swell to break the bay ice. That night we saw her forecastle lanterns, and by noon the next day she had tossed a kedge anchor to a passing ice pan. You could hear the men singing as they pulled in their mooring lines and slipped out for the icefields to the northwest and into the blue drop."
The unabridged audio edition of The Big Why is available from rattlingbooks.com: