What is the black spot I see in a low powered eyepiece in my reflector during daylight?

A low powered eyepiece in a reflector produces a large exit pupil with a large image of the secondary mirror obstruction. During the day, when the pupil of the eye is small, if the size of the secondary obstruction image approaches the size of the pupil, it will appear as a darkened region in the center of the field. At night, when the pupil of the eye is large, the darkened region is not noticed.