Obviously, the geometry of the Moon's path through the umbra plays an important role. Not as apparent is the effect that Earth's atmosphere has on eclipses. Although the physical mass of Earth blocks off all direct sunlight from the umbra, the planet's atmosphere refracts some of the Sun's rays into the shadow. This material filters and attenuates the sunlight before it's refracted into the umbra. For instance, large or frequent volcanic eruptions dumping huge quantities of ash into the atmosphere are often followed by very dark, red eclipses for several years.
Extensive cloud cover along Earth's limb also tends to darken the eclipse by blocking sunlight.
Moon almost invisible, especially at mid-totality. Details distinguishable only with difficulty. Deep red or rust-colored eclipse. Very dark central shadow, while outer edge of umbra is relatively bright. Umbral shadow usually has a bright or yellow rim.