Comments: The PAT form is a compound *jǝkʷa-c̣ʷa (for the first part cf. Abkh. a-jkʷá dark'). -c̣ʷa can not be treated here as the same root as PAT *jac̣ʷa 'blue, green' (q.v.) for accentological reasons: a-jkʷá(j)č̣a would be expected in Abkh.).
The PAK word *c̣ʷǝc̣a 'black' goes back (as proposed by Шагиров 2, 103) to *c̣ʷǝ 'black' + *ṗc̣a- 'thick, dense' with secondary elision of -ṗ-; the root *c̣ʷA 'black' is used also in a number of other compounds.
Since exactly the same correspondence is present in the word for 'fire' (q.v.) one can not separate the Ub. form from the rest, but problems remain: we have to account for deglottalisation and delabialisation of the root consonant. We can, however, suppose that the original form is still preserved in the word for 'coal': Ub. ǵá(n)-c̣ʷa, cf. PAK *c̣ʷa-mǝč̣́ǝ id.