If you already have a Black Arrow buff and are standing next to multiple Black Arrows on the ground when you consume you current buff (I would do this on purpose so I could fire multiple buffed attacks in succession), all of the nearby arrows on the ground are consumed simultaneously, even though you can only have one buff at a time.
Sometimes this results in you having just the one new buff, effectively deleting the extra arrows. Sometimes it results in no buff at all, as if the arrows canceled each other out, effectively deleting all arrows that were on the ground near you.