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
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Throwing knives used by executors of the Holy Church. The blade is knit from magecraft, so the user is only really carrying around the handle.

Fate/Zero Animation Guide I: Glossary of the holy grail war I
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Conceptual armament adopted by the Holy Church. Since they are seen a lot they are thought to be a basic armament for every agent of the Church but that’s not the case, it’s very difficult to handle, and a weapon with suppressive power used for the first time by the expert class. In the Church of nowadays, where the usage of modern weapons is allowed, people stepping back and saying “Wow, is that agent using Black Keys…? They’re a training maniac. They’re a training maniac” are often seen.
Shirou, who was losing his temper for not being able to break the Ainsworth’s Bounded Field no matter how much time passed, when he urged ‘Anything is fine so lend me a conceptual armament, you have something good, huh?,’ they were sold to him at a price with which he could buy a new game console.
By the way, in the original work Fate they are given away for free. It may be for the difference of relationship between both, or due to the difference of intention, but more importantly the reason may be because this Kotomine is a “merchant” for the time being. The one who runs the store is severe about money payments. It’s not like he will never overcharge. The price of ramen? Market value.

PRISMA material/petit: Encyclopedia of Prisma Illya MOVIE