黒風烟嵐 sits at the darkest end of Sanemi's naming palette: every other numbered form describes a wind that's visible and almost picturesque (dust funnels, clearing mist, withering leaves) — this one describes a storm that blacks out the air entirely, smoke-dark and disorienting. That matches its narrative placement: it's thrown mid-battle against Muzan at a moment where visual confusion, not raw cutting power, is the tactical goal, echoing the same reconnaissance-through-obscuration logic already noted for Form One's dust funnel.
VIZ's insertion of "Mountain" into "Black Wind Mountain Mist" is the more interesting translation choice here, since no single kanji in 黒風烟嵐 denotes a mountain. The likeliest explanation is stylistic continuity with the immediately preceding numbered form (木枯らし颪, "Cold Mountain Wind") — VIZ's translators may be building a small in-house naming rhythm across consecutive Wind Breathing forms rather than translating each name in total isolation, trading a little precision for a run of titles that read as a coherent set to an English audience encountering them back-to-back.