A very interesting common NC root. The original meaning 'dark, blue' in several subgroups developped to 'blue metal' > 'iron' (on the possible explanation of this semantic change see above, in the WC section). The PN form meaning 'iron' (*ɦāχḳi) can be explained best of all if we suppose a laryngeal prefix : *nHǟƛ̣_wV̆ 'blue' - *h-nǟƛ̣_ū (or *h-ǟnƛ̣_wV̆) 'blue thing, metal' (cf. the same archaic derivational model in the pair *mĭʒ_V 'sweet' - *H-mĭʒ_ū 'sweet thing, honey'). In that case we can also relate here (from the derived noun) a local EC root meaning 'horseshoe' (the semantic development is natural: cf. numerous cases like Darg. meχ 'iron; horseshoe' etc.): PTs *heƛ(u) A: Tsez., Gin. hiƛu (PTsKh *heƛu); Gunz., Bezht. heƛ (PGB *heƛ); PL *Häƛ̣:(a), reconstructed on basis of a relic form: Rut. äg 'horseshoe'.
Finally, another derivate from the same root may be PEC *nɦäƛ̣_wV 'gadfly, horsefly' (possibly as a "blue insect") q.v.
See Trubetzkoy 1922, 241 (Lak.-Arch.).