The full moon is the secondary source of light upon the earth (Genesis 1:16). That it now turns deep red (like blood) means that it fails to supply light, and it now also symbolizes death on the earth (see Revelation 6:4). With the opening of the sixth seal, this secondary light source created on the fourth day also loses its ability to function (see also Joel 2:31).
12 When he opened the sixth seal, I looked, and behold, there was a great earthquake, and the sun became black as sackcloth, the full moon became like blood,