Here's why it wasn't a dagger:
* Description: The blade is described as "long and black as night, and cold" and having "a sharp edge that bit like frost" (The Fellowship of the Ring). This doesn't fit the typical description of a dagger.
* Purpose: The blade wasn't used to kill Frodo but to inflict a morally corrupting wound. This is why the wound was slow-acting and filled Frodo with a yearning for the One Ring.
* Magic: The Nazgûl's weapons were infused with their dark magic, making them more potent than simple blades.
It's important to remember that the Nazgûl weren't purely physical beings. Their weapons were infused with their corrupted nature, making them more than just ordinary weapons.