Black Key

No, not the black keys on a piano. The throwing swords wielded by Ciel.
As for why they're called black keys, refer to the entry on purification.

Tsukihime Data Collection: Tsukihime Dictionary
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No, not the black keys on a piano. The throwing swords wielded by Ciel.
As for why they're called black keys, refer to the entry on purification.
Ciel carries a large number of these concealed on her person, but what's under her robes is actually just the hilts. She materializes the blades by passing magical energy through pages of the bible. The maximum number she is able to conceal at one time is close to 100. Also, since black keys are designed specifically for throwing and not for swordplay, in a way they could be considered a kind of bullet.
Though they are counted as one of the basic weapons of the Church, the fact that they're relatively hard to use means that very few executors favor them.

Tsukihime Dokuhon Plus Period: Tsukihime Dictionary Revised
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The throwing swords wielded by Ciel.
Weapons for use against vampires, monsters that distort the laws of nature. These “keys of providence” call forth the original laws of nature the vampire was subject to as a human.
Ciel carries a large number of these concealed on her person, but what's under her robes is actually just the hilts. She materializes the blades by passing magical energy through pages of the bible. Because the black keys are quite compact before this transformation, the maximum number she is able to conceal at one time is close to 100.
Despite their appearance black keys are not designed for swordplay, but are in fact throwing weapons. The Burial Agency even has a secret technique called the Iron Plate Effect that allows them to throw more effectively. Because Ciel has mastered this technique, her black keys are capable of boring through concrete walls.
They are counted as one of the basic weapons of the Church, but the fact that they're relatively hard to use means that very few executors favor them.
Also, besides the Iron Plate Effect, there are other secret throwing techniques such as the Cremation Rite and Internment Rite. The Cremation Rite, in particular, is used in Ciel's Last Arc.

MB Act Cadenza PS2 Manual: Tsukihime Dictionary