Basically the Sable Church has a loose connection to the deity Velka from DS1, who herself has some connection to humanity and therefore darkness. Faith is a stat that canonically connects an undead to the power of the Gods, so it makes sense why this is a miracle and scales only with the spell buff from Faith. Also, that's not true. Another weapon buff that only scales with Faith is the Lightning Blade miracle for the same reason. A lot of this stuff has never been fully pieced together and the narratives are pretty convoluted, so don't worry too much about it.