why are clouds grey...not blue?

[whowhatwhy]

Ok I know that this is probably an obvious answer, but today I was looking at a huge, almost black in appearance thunder cloud, and thought 'why is is grey / black'? I know why the oceans are blue (appear blue) I know why water appaers blue from reading answers on this website. So why does water in the atmosphere in the form of big heavy clouds appear dark grey, and not blue?

[Clanad]

The thickness of the clouds (especially cumulonimbus) inhibits passage of light through them, making them appear dark, even sometimes black. If you could fly near them at their same level, they often appear snow white, due the refraction of the full spectrum of light...