Amara from Supernatural was the Darkness. When she was unleashed she was a cloud of black smoke/fog.
But in Season 11 Episode 20 "Don't Call Me Shurley", her fog that infects people is white.
Why would her infectious fog not be dark like herself?
The fog wasn't white - it was grey. The black smokes (or black stuff) seems to represent more intense form of darkness whereas the grey version seems to represent less intense version. This can be (in part) supported by the fact that the God could very easily overpower the effect of the grey fog but couldn't defeat the darkness when darkness was in her full power.