كما في تشبيه الثوب الأسود بالغراب في شدة السواد
As in comparing black cloth with a raven for the intensity of its blackness.
The raven serves as a familiar measure because its black colouring is conspicuous to the audience. The cloth's being black may already be known; the comparison is now intended to make the degree of blackness more readily imaginable.
If the listener has no sense of the comparison term's colour, the illustration fails. Likewise, if the real issue were whether the cloth is black at all, the same words might be functioning to show the condition rather than its amount. Audience knowledge and communicative purpose govern the classification.