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The Egyptian hieroglyph for "black" in Gardiner's sign list is numbered I6. Its phonetic value is km. The Wörterbuch der Aegyptischen Sprache-(Dictionary of the Egyptian Language) lists no less than 24 different terms of km indicating 'black' such as black stone, metal, wood, hair, eyes, and animals.
The most common explanation for the hieroglyph is under the Gardiner's Sign List, section I for "amphibious animals, reptiles, etc" is a crocodile skin with spines. Rossini and Schumann-Antelme propose that the crocodile skin hieroglyph actually shows claws coming out of the hide.
Besides 'black', the alternate use of the hieroglyph is for items terminating, coming-to-an-end, items of completion, hence a reference to charcoal, burning to its ending.