What's a "black key"?
Black Keys are a type of conceptual weapons (albeit weak), called Keys of Providence. They are essentially charms used by agents of the Church to exorcise demons. They are meant to bind and/or seal spiritual phenomena (e.g. Vampires hunted by the Church), rather than hurt them.

The Type-Moon Wiki notes that: 


  If they hit the shadow of a target, the target will be unable to move their body, leaving them to the mercy of the black key's wielder. It could also be used to track down and seal or injure a target until its wielder tells it to stop doing so, or unless it is destroyed or neutralized by an outside force. 


It's been noted that they can take the form of many types weapons, but are usually in the form of long, rapier-like swords or spears, which they are generally thrown as a dagger or used to stab opponents. Ciel-senpai and Kotomine Kirei are notable wields of these weapons.

Sacraments, which are basically spell-sigils, can be engraved in Black Keys to give them different effects.