Rotten ice

From Glossary of Meteorology

rotten ice

Any piece, body, or area of ice that is in the process of melting or disintegrating.

It is characterized by honeycomb structure, weak bonding between crystals, or the presence of meltwater or seawater between grains. Rotten ice may appear transparent (and thus dark) when saturated with seawater and so may be easily confused with newly forming black ice.