"Well blow me down!"
As an oath, 18th c.

1867, Clark, "Seven Years of a Sailor's Life," : "Hallo, Charlie, blowed if I saw you before."

Lighter (Historical Dictionary of American Slang) accepted the 'knock-down' interpretation, but his earliest (1886) quotation is from an article that makes no comment about the meaning.
Jour. Amer. Folklore- "Blow the man down in the hold below."

Lyr. Add: BLOW A MAN DOWN
Hatfield, coll. 1886 'Ahkera'

O 'low me some time to blow a man down!
Too ma hay ho, blow a man down.
Blow the man down in the hold below,
O give me some time to blow a man down!

From starboard to larboard away we will go!
(Too ma hay ...)
From larboard to starboard away we will go!
(Give me ...)

O, hip, hip, hip, and away we will go!
(Too ma hay ...)
We'll rise and shine and ma-ake her go!
O Give me some time to blow a man down!

"Possibly the most widely used shanty. The melody was practically the same in all versions, which usually had much longer, and widely divergent texts."
"The working-crew consisted of eight strapping Jamaica Negroes." The crossing was from Pensacola to Nice, France, 84 days.
With score.
Hatfield, James Taft, "Some Nineteenth Century Shanties," Jour. American Folklore, 1946, vol. 59, no. 236, pp. 108-113.