slate-blue

The compound adjective 'slate-blue' is a straightforward descriptive term from the late 18th century, formed by combining the name of a material with a colour. 'Slate', the fine-grained rock known for its characteristic dull grey-blue hue, is used attributively to modify the colour term 'blue'. This follows a common English pattern for creating colour names by referencing natural objects, such as 'sky-blue' or 'rose-red'. The logic is purely comparative, evoking the specific shade by directly linking it to the familiar appearance of the sedimentary rock.

Structure: slate (rock) + blue (colour) - slate: From Old French esclate, meaning 'splinter, fragment', referring to the rock that splits into thin layers. In this compound, it functions as a modifier specifying the type of blue. - blue: From Old French bleu, of Germanic origin, referring to the colour. In this compound, it functions as the head noun denoting the base colour.