"The Oxford English Dictionary itself feebly admits that 'In Middle English it is often doubtful whether blac, blak, blacke, means "black, dark," or … - Mark Forsyth

"The Oxford English Dictionary itself feebly admits that 'In Middle English it is often doubtful whether blac, blak, blacke, means "black, dark," or "pale, colourless, wan, livid".'

...

Utterly illogical though all this may sound, there are two good explanations. Unfortunately, nobody is quite sure which one is true. So I shall give you both.

Once upon a time, there was an old Germanic word for burnt, which was black, or as close to black as makes no difference. The confusion arose because the old Germanics couldn't decide between black and white as to which color burning was. Some old Germans said that when things were burning they were bright and shiny, and other old Germans said that when things were burnt they turned black.

The result was a hopeless monochrome confusion, until everybody got bored and rode off to sack Rome.

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The other theory (which is rather less likely, but still good fun) is that there was an old German word black which meant bare, void, and empty. What do you have if you don't have any colours?

Well, it's hard to say really. If you close your eyes you see nothing, which is black, but a blank piece of paper is, usually, white. Under this theory, blankness is the original sense and the two colors — black and white — are simply different interpretations of what blank means."