Why does newly formed lake ice sometimes look black and transparent?

Newly formed lake ice appears black and transparent because of the specific orientation and size of the initial ice crystals that form on the water's surface.

Under ideal conditions—typically on a cold, still night—water at the surface cools to its freezing point and becomes slightly supercooled. This triggers the rapid propagation of ice crystals across the surface. Unlike the horizontal orientation of crystals that develop during later stages of ice thickening, these initial crystals are oriented with their c-axes vertically. Under these conditions, these crystals can reach dimensions of one meter or more, creating a cover that is highly transparent and appears black against the dark water beneath.1

This phenomenon is distinct from other forms of ice, such as frazil ice, which forms in turbulent rivers and often appears as slushy or opaque masses. While pure ice is naturally transparent, the presence of air bubbles can render ice somewhat opaque, which is why the clarity of this initial "black ice" is notable.23