blue field

Blue field most often describes an expanse of sky or water characterized by its blue color: We picnicked under a blue field of cloudless sky.

In heraldry, a blue field is the background of a coat of arms rendered in azure.

The term also appears in vision science, where the blue field entoptic phenomenon refers to seeing tiny bright dots moving across your visual field when looking at a uniform blue surface—actually white blood cells moving through retinal capillaries.