"Zuraqa" most likely refers to blue, the last visible color of a dimming flame. In wood flames the hue comes from carbon and hydrogen which emit in the blue and violet.
Dr Shabbir Ahmed (Quran As It Explains Itself)
20:102 The Day when the Trumpet is blown - On that Day We will assemble the guilty with their eyes dimmed in fear. [20:124]
Alluding to eyes dimmed blue in fear of retribution.