Wiktionary
blue spot
n. (context snooker English) The spot on a snooker table where the blue ball is normally placed.
Usage examples of "blue spot".
A blue spot burned down on her face buried in the pillow, turning her taffy-honey hair a ghostly white.
It was a little more luminous where Robert Lorrey stood, and Renny distinctly saw Lorrey's head kick back and a small blue spot appear in the center of his forehead.
On his forehead, exactly between his eyes, was a blue spot that suddenly trickled red.