The darkness spell can be cast on small objects, and can be covered, but can it be cast on the blade of a dagger, and covered by sheathing the dagger and leaving the rest bare?
Or do I have to cast darkness on the entire dagger, and keep the dagger covered in a cloth or pocket?
Alternatively, are pieces of whole objects valid targets for spells? What defines an "object"? Is a blade separate from a hilt? Is my ring separate from my hand?
Best Answer
Yes; darkness targets a point, not an object
From the description of the darkness spell:
As you can see, darkness does not target the object itself; it targets a point you choose within range, which can be on an object. If you covered the entire object except for that one point, the darkness would still emanate and would not be blocked. On the other hand, if you block that one point on the object, then the darkness is blocked even if you don't block the rest of the object.
In your example: you could cast darkness on a point on the blade of your dagger and then sheathe it, which would block the darkness from emanating from that point until uncovered.
Note that for the second paragraph of the spell description to apply, it has to be a point on either an object you are holding, or an object that isn't being worn or carried. Technically, those parts of the spell would not apply if you cast the spell on any other point.
(To clarify why I say darkness originates from a point on the object rather than the object as a whole: The entire object can't be "a point" that the darkness emanates from. The point of origin of the spell is a single point. If that point is on an object you're holding or an object that's not worn/held, then the point of origin "sticks" to that point on the object - but the entire object can't serve as the spell's single point of origin.)