What is obtained by adding black to blue-green?

Correct answer: A darker shade of blue-green

The governing concept is shade formation. When black is mixed with a colour, the colour’s value decreases, producing a darker version called a shade. Adding black to blue-green therefore gives a dark blue-green shade while preserving its general hue family. It does not make the colour brighter, convert it into pure blue, or produce the unrelated tertiary colour red-orange.