Dark is one of the basic elemental attributes found in Castlevania. It is often associated with Dracula and his powers. In Symphony of the Night, though not officially, Alucard can use some of Dracula's dark powers such as Hellfire, "Soul Steal" (HP drain version), Animal shift and others. Death officially wields dark damage with others of vampiric or demonic relations. The only exceptions are possessed characters like Hugh Baldwin and Richter Belmont. Due to Soma Cruz being Dracula's reincarnation he has access to some dark-based souls and powers.
There are weapons, armor, and some forms of magic that possess dark attributes, either due to their origin or their use. Examples of these weapons include Death's Scythe, Kaladbolg, Umbra, Voulge, Dark Gate, and surprisingly, Cream Pie. Despite dark weapons seem to have tempting effects, the fact that most enemies of the game are resistant to it makes them much less effective in most situations, especially there are only a handful of holy enemies out there. However most games contain at least 1 holy based boss, usually a hero-turn-villain boss which is difficult to defeat while using conventional weaponry. This is where the dark weapon comes to handy.and it might help matters.