That's a mere blind. A pretence; something ostensible to conceal a covert design. The metaphor is from window-blinds, which prevent outsiders from seeing into a room.
Blind as a bat. A bat is not blind, but when it enters a room well lighted, it cannot see, and blunders about. It sees best, like a cat, in the dusk. (See Similes.)
Blind as a beetle. Beetles are not blind, but the dor-beetle or hedge-chafer, in its rapid flight, will occasionally bump against one as if it could not see.
Blind as a mole. Moles are not blind, but as they work underground, their eyes are very small. There is a mole found in the south of Europe, the eyes of which are covered by membranes, and probably this is the animal to which Aristotle refers when he says, “the mole is blind.” (See Similes.)
Blind as an owl. Owls are not blind, but being night birds, they see better in partial darkness than in the full light of day. (See Similes.)
You came on his blind side. His soft or tender-hearted side. Said of persons who wheedle some favour out of another. He yielded because he was not wide awake to his own interest.
“Lincoln wrote to the same friend that the nomination that the democrats on the blind side.” —Nicolay and Hay: Abraham Lincoln, vol. i. chap. xv. p. 275.
Blind leaders of the blind. The allusion is to sect of the Pharisees, who were wont to shut their eyes when they walked abroad, and often ran their heads against a wall or fell into a ditch. (Matt xv. 14.)
The Blind Francesco Bello, called Il Cieco.
Luigi Grotto, called Il Cieco, the Italian poet. (1541-1585.) Lieutenant James Holman, The Blind Traveller. (1787-1857.) Ludwig III Emperor of Germany, L'Aveugle. (880, 890-934.)
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894