After some extensive Googling I have come to the conclusion that no-one really knows fer shewer, so some informed guesswork is used. Some people have said that a very deep brown (almost black) was achieved using IO's oak, some people say they got black by mixing three other (very expensive) dyes and some people say they probably couldn't have achieved a true black with the dyes at their disposal. Presumably they didn't have gentle washing machine detergents either.
When is black really black? A very dark colour that looks kinda black would be close enough for me.
But they didn't call it black.
So they could have sung 'Blue Moon', meaning a total eclipse presumably ... and 'Don't it Make my Brown Eyes Blue' could have been about getting a thump on the head.

