The dark appearance of the underside of a cloud layer when it is over a surface of open water; it is darker than land sky, and much darker than ice blink or snow blink

https://nsidc.org/cryosphere/glossary/W

By Mabra99 - Self-photographed, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=25766283