User blog comment:Guildmaster Grovyle/Character Chart: Pokémon/@comment-27161581-20160216154547/@comment-7345250-20160216162959

I was trying to account for this.

The original plan was Dark/Rock, but whereas the Crystal ponies are creatures of light and clarity, the Umbrum are creatures of shadow and smoke. All things considered, Ghost seems far more appropriate than Dark in Sombra's case. Rock accounts for his crystal magic too and syncs well with Ghost.