The “Dark Side” of the Moon refers to the hemisphere of the Moon that is facing away from the Earth, first seen with their own eyes by the crew of Apollo 8.
In reality it is no darker than any other part of the Moon’s surface as sunlight does in fact fall equally on all sides of the Moon. It is only “dark” to us, as that hemisphere can never be viewed from Earth due to a phenomenon known as “Tidal Locking”. A better term for the side we don’t see is the “farside’” rather than the “dark side.” Borman took these very rare photographs on orbit 8 with the 250mm telephoto lens as he enjoyed a lonely view of the spectacular backside of the Moon during Lovell and Anders’ sleep period.
“We were like three school kids. The most awe-inspiring sight. Looking back at the back side of the Moon [...] for the very first time.”
—James Lovell (Chaikin, Voices, pg. 36)
The first, superbly detailed vertical photograph shows the floor of the 37-km Crater Planté (10°S latitude and 163°E longitude, unnamed at the time of the mission) located near the eastern inner wall of the much larger 160-km Crater Keeler (also unnamed at the time of the mission). The area covered by the photograph is approximately 20 miles (32 km) on a side.
The second photograph offers a striking oblique view of Tsiolkovsky, the most prominent crater on the farside, looking southeast showing its central peak and dark mare floor.
Tsiolkovsky is a large, 200-km crater on the far Moon’s side which was first photographed by the Soviet probe Luna 3 on 7 October 1959. In the poor imagery of the time, its dark, mare-like interior made it stand out from the other craters that pepper the farside. The triumphant Soviets, in the manner of all explorers, promptly and appropriately named it after the Polish-Russian pioneer of spaceflight theory, Konstantin Tsiolkovsky (1857-1935). A dominant, W-shaped central peak of light colored highland material rising out of the dark material makes this crater particularly distinctive and striking. (from the Apollo 15 Flight Journal at 083:16:51)