To be honest, when I first looked at the five colors that was my natural reaction too. I have a personal hypothesis that comes right away from my computer graphics experience but I'll need professional Sinological input to confirm my experience.
The Chinese liked elemental groupings of five - five tastes, five elements. Since their astrological theories required five colours, they used the three subtractive primary colours (red, blue yellow) and white and black that are the natural endpoints of the colour scale. Having no other element to associate with black (since the other four associations come naturally) the association of water with black was made.
There might also be a thematic reason for this: water in Chinese culture has an intrinsic property of chaos (indeed, in all cultures) given that the predominant disaster of primitive Chinese civilization on riverbanks would have been annual flooding. While black has an obvious connection to evil. The connection between water, tragedy, evil, and black may have been obvious.
(And who says water can't be black? Remember Dark Water? XD)
Besides, there has been nothing at all pertaining to issue #3 on my OP.
(Frustrating! My library's academic journal subscription only has a citation of Ceragioli, not the full article! Sheesh.)