What causes clouds to appear blue? In a thunderstorm cloud about sunset time, I saw these clouds, including some (in the upper right) that were a unique shade of blue.  I don't think I've seen clouds quite that color before.  I tried to capture the color as best I could with a digital camera, though the blue is slightly washed out in the image.  I took these images facing East at sunset time, during summer.



Optically zooming in on the blue cloud:


This answer suggests that the sunlight should be missing those wavelengths ("at sunset, when only the red/yellow end of the spectrum hits") which is consistent with the brilliant warm colors to the left.  But...the cloud was a strong and unique shade of blue, different from the blue behind it.  How does this happen?  That is, what natural processes underly the occurrence?

(Although that question is on Physics.SE, there seem to be more questions about cloud appearances on this site.)
The true reflected colour of a cloud doesn't change from white unless there is serious pollution, such as Saharan dust, wildfire soot or industrial emissions. None of these produce a blue colour, so the question really becomes 'what reflected light looks blue', and that is a function of the direction of view (towards or away from the sun), the subject's perception of colour (which can be surprisingly deceptive), and how much atmospheric blue back-scatter the cloud is reflecting. The latter accounts for most of the 'blue cloud' that we see.