Chevalier d'Eon
Rapier and Dagger
The Heroic Spirit of the Sword (rapier) Chevalier d'Eon looked in confusion at the bluish hilt - and nothing else- that Chaldea's First Master had eagerly presented him with.
"Gudako, what exactly is this?" An educated guess said some sort of Mystic Code or other such tool, probably a weapon but he already had one.
"It's a Black Key," she replied immediately. "Or rather, it looks like one, but it's really the crystallization of the concept of 'a weapon that strikes swiftly, with deadly accuracy'. I think it'd suit you."
"Where would you even get something like this? Actually, how would I use it, I'm not a magus?" It felt heavy for a mere hilt of its size, but had no obvious markings to suggest how it was activated.
"You're a Servant, so channel some of your prana into it and fix in your mind what form you want." Doing so resulted in a long narrow thrusting blade. d'Eon frowned and knocked the tip against the wall next to him.
"Oh. I thought you'd want it more like a main-gauche," she commented, voice slightly tinged with disappointment. "Wasn't rapier and dagger the preferred form in your time?"
"Ah," he said, remembering. "Now that you mention it, I wonder why I wasn't summoned with this sword's partner."
As the Black Key's blade shrank to that of a long dagger, d'Eon transferred it to his left hand in order to draw his scabbarded weapon and take a fencing stance.
"Probably didn't have enough of a legend. Well let me know how it goes."
Notes
Not a really serious series. It's not the same Chaldea as canon FGO, but anyway this is just focusing on Servants who might find use for Black Keys.
Though they're CEs you equip (and look like a set of three) in these stories each "summon" is a single Black Key with slightly different concept of a weapon, corresponding to the trio of Quick (Verdant), Arts (Azure) and Buster (Crimson).