According to Tobias Capwell black armour was far more common than we now think. It was the mirror polished armour that was far rarer - polishing took more man hours than fabricating the armour and thus was very expensive - hence the 1472 painting of Federico da Montefeltro, Duke of Urbino by Piero della Francesca which depicts the Duke in what we would consider relatively plain and lacking décor these days. I would add that this is also very difficult to maintain blemish/rust free (personal experience of owning and wearing my own armour - many many hours of re-polishing)
Black from furnace, painted or fabric covered was far more common as research into extant items at the Wallace Collection suggests. It was the Victorians that developed the "knight in shining armour" image which stays with us today. That said, I will continue to paint my 28mm medieval armies with polished as it looks right and I would add that Simon's painting in this field is well researched, a joy to see and inspirational.
Grant