There could be several reasons why the sky appeared to get dark for a quick second. Here are a few possible explanations:
Passing cloud: If a dense cloud passes in front of the Sun or the source of light, it can momentarily block the sunlight and cause a brief period of darkness. This can happen even on a relatively clear day if a cloud happens to pass by at the right moment.
Bird or aircraft: Sometimes, the shadow of a bird or an aircraft flying overhead can momentarily cast a shadow on the ground, causing a localized darkening of the immediate area. This effect is typically brief and lasts only for a few seconds.
Solar eclipse: During a solar eclipse, the Moon passes between the Earth and the Sun, partially or completely blocking the Sun's light from reaching certain areas on the Earth's surface. If you happen to be within the path of a solar eclipse, you may experience a brief period of darkness as the Moon blocks the Sun.
Atmospheric conditions: Atmospheric phenomena such as a passing dust cloud, localized haze, or pollution can also temporarily reduce the amount of light reaching the ground, causing a momentary darkening of the sky.
These are just a few possible explanations for a momentary darkening of the sky. The specific cause would depend on the circumstances and the observations made at the time.