This was, at first, a Yiddish proverb:
“At night all cows are black.”
בײַ נאַכט זענען אַלע קי שוואַרץ. (Hebrew from this site, sorry if it is incorrect)
The implication was that, at night, all women are the same. So don’t worry too much if you’re sleeping with an ugly girl or some one who is not your wife. It is roughly analogous to the English proverb, “When the candles are out, all women are fair.”
But, in 1806, Mr. Georg Wilhelm Friedrich Hegel used the expression in the Preface to his revolutionary Phenomenology of Spirit:
“To pit this single assertion, that “in the Absolute all is one,” against the organized whole of determinate and complete knowledge, or of knowledge which at least aims at and demands complete development – to give out its Absolute as the night in which, as we say, all cows are black – that is the very naïveté of emptiness of knowledge.” (Hegel is difficult to understand in short quotations like this one.)
Hegel changed the significance of the cows from ladies to be a metaphor for that which cannot be known or distinguish. At the time, Romantic thinkers such as Hölderlin and Schelling were holding sway with the idea that higher truths can only be grasped through intuition or feeling. For an older, more explicitly Catholic, rendering of the same philosophy, see the last few cantos of Dante’s Paradise (Canto 33, lines 82-96)
in which Dante describes “the universal form,” meaning the totality of wisdom, as “the fusion of all things.”
It is just this totality that Hegel rejects. In his mind, absolute wisdom, or any wisdom at all, must be found through distinguishing, differentiating and defining. Thus it is the empirical mind that reasons clearly, not the heart that feels vaguely, that brings us to truth.
With Hegel’s interpretation of the expression, “black cows at night” became a kind of refutation of the idea that nothing could be known.
In his 2007 essay, “The Absolute and the Relative,” Umberto Eco uses the expression to describe how some think of the ultimate absolute, meaning god, whose existence is contingent upon nothing other than itself.
It is an idea that we can easily imagine, meaning we can make a picture of it, but it gives us very little insight into either the cows or the night.
Obscure Reference 
I have used this phrase: “At night, all cows are black” as an expression meaning that the observer doesn’t have all the data, thus the analysis is likely incorrect. (Or, more that the analysis could be true or false. ) It’s a handy phrase to have in your back pocket, as long as you’re not in a Yiddish area 😉. It is a phrase that makes people think about what was just said and how it might apply. In other words, it’s a little like saying “well, yes and no”.
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