Answer:
A single syllable do I claim black was my most famous name; Fetal to mortals here below thousands have I slain in a single blow.
What gets sharper the more you use it?
I’m found in your socks, scarves, and mittens; and often in the paws of playful kittens. What am I?
What goes up and down, but does not move?
I go in hard come out soft. Blow me hard and I’ll make a pop.
Whoever made me don't want me. Whoever bought me don't need me. Whoever use me don't know me.