What causes the moon to be darkened in a lunar eclipse?

the moon does not have any light of its own it shines because its surface reflects sunlight. during a total lunar eclipse the earth moves between the sun and the moon and cuts of the moon's light supply when this happens the surface of the moon takes a reddish glow instead of going completely dark

The sun rays are blocked from reaching the moon.

Explanation:

The earth lies in between the sun and the moon.

Since the earth is in the middle, the sun rays are blocked from reaching the moon, hence dark.