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.
In the Demotic (Egyptian) text of the Rosetta Stone, the demotic for Egypt is 'Kmi' . There are three uses of the actual Kmi, but 7 others referenced as Kmi refer to iAt in the hieroglyphs. Other euphemistic references to Egypt in the Rosetta Stone include "Ta-Mer-t", which has the meaning of the 'full/fruitful/cultivated land', hr-tAwy, the 'lands of Horus', and tAwy, the "Two Lands."
Coming to a conclusion, or completion is one use of the km hieroglyph in the words kmt and km iri ('to make an end'). The discussion of the biliteral states: The conclusion of a document, written in black ink, ending the work, has the same semantic connotation. (as km for 'concluding') The Rossini, Schumann-Antelme write-up states that initially the word comes from "shield", ikm, and thus the original association with the crocodile.
Since the origin of the 'black' hieroglyph, and relationships to other forms showing vertical flame-like rises on the end of a flat-triangular shape, there has been discussion of this hieroglyph. As stated above, Schumann-Antelme and Rossini explain the crocodile skin, but with claws. The text by Egyptologists Mark Collier and Bill Manley describes the same "crocodile skin", Gardiner I6, as burning charcoal with flames.
The last three of four entries ending the 27 entries deal with black stones, or powders and black plants, or seeds; (all small multiple, plural, grains-of, items). They are preceded by entries 21 and 22, a "buckler", or "shield", and "black wood". Entry 26 is an image, or statue, using the vertical mummy hieroglyph gardiner A53, ("in the form of", "the custom of"). These last six entries are unreferenced.