[philwelch]

A "full Earth" would coincide with the sun fully illuminating the far side of the Moon, and a "new Earth" would coincide with the sun fully illuminating the near side of the Moon, so wouldn't you only get maximum darkness anywhere on the Moon during a lunar eclipse?

However, counterintuitively, I think you'd be better off with a full Earth and no direct sunlight than a new Earth and full direct sunlight, making the near side a slightly better place for an optical telescope. The near side of the moon even has the opportunity to experience eclipses.