Sackcloth is black in colour and so it is a fitting image for what the turned-off sun would look like. Commonly the sun is a source of light, being anything but black (dark, without light). With the opening of the sixth seal, the light source God created on the fourth day (Genesis 1:14–18) will lose its characteristic ability (see also Isaiah 13:10; Isaiah 50:3; Joel 2:31). With the sun’s demise comes also a loss of the sense of time (cf. Genesis 1:14b).
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,