The premise of your inquiry highlights a fascinating optical and physical challenge: during a solar eclipse, from the vantage point of the Moon, the Earth appears almost completely "Full." The Earth is roughly four times the diameter of the Moon and has an albedo nearly three times higher due to clouds, water, and ice. This means that during a solar eclipse, the Earth-facing side of the Moon is bathed in the maximum possible intensity of Earthshine—vastly brighter than the faint Earthshine visible on a thin crescent moon in the night sky. The reason this illumination seems invisible or difficult to detect from the ground during an eclipse comes down to the mechanics of optical dynamic range, sensor saturation, atmospheric scattering, and optical filtration.

During the partial phases of an eclipse, as the lunar body traverses the Sun, the uncovered portion of the solar photosphere remains blindingly bright. The surface brightness of the Sun is roughly one million times greater than the surface brightness of the Earthshine-lit Moon. To safely observe or photograph the traversing edge without damaging an eye or frying an optical sensor, one must use a high-density solar filter, typically an ND5.0 filter, which attenuates incoming light by a factor of  or 99.999%. Because the faint Earthshine hitting the Moon is already thousands of times dimmer than direct sunlight, passing that light through a heavy filter extinguishes it entirely, rendering the Moon as an absolute black silhouette. Furthermore, if you were to remove the filter during the partial phase to search for Earthshine, the direct solar glare would flood the camera barrel or telescope tube, causing massive internal lens flares and atmospheric forward-scattering that instantly washes out the faint silhouette.

Once totality is achieved and the solar photosphere is fully blocked, the heavy solar filters are removed, and the physical Earthshine on the lunar surface becomes directly accessible to optics. However, the human eye still struggles to see surface features for two reasons. First, the solar corona emerges with intense radiance; the inner corona alone has an integrated brightness comparable to a full moon, concentrated in a sharp, high-contrast ring directly bordering the black lunar limb. The human eye cannot simultaneously adjust its iris and retinal response to resolve both the brilliant pearlescent corona and the faint, low-contrast lunar maria right next to it. Second, totality does not produce a pitch-black midnight sky; it creates a 360-degree horizon twilight equivalent to deep dusk, which establishes a background sky luminance that competes directly with the low surface brightness of the dark lunar regolith.

Despite these severe contrast limitations, the Moon's face during totality is not actually a featureless void on a sensor. Modern astrophotographers using long, unfiltered exposures and bracketed High Dynamic Range (HDR) processing regularly resolve the lunar surface features during total solar eclipses. By capturing exposures ranging from fractional seconds for the inner prominences up to two or three seconds for the dark disk, and digitally stacking them to compress the extreme dynamic range, the familiar albedo patterns of the lunar maria—including Oceanus Procellarum, Mare Imbrium, and Mare Serenitatis—become clearly visible within the silhouette, illuminated entirely by the light bouncing off the sunlit continents and clouds of the Earth.