why isn't phlegm air it's literally in the respiratory system (because the four humors work on a hot/cold wet/dry grid and water and phlegm are colder than air and blood I guess)
yeah I've heard a lot of different explanations for them, I think yellow bile being actual bile/stomach acid is the usual theory, which makes sense. but black bile is harder, sometimes they say it's when other fluids etc get dark, or a mixture of them, or cancerous/diseased tissue...