A blunt object is rounded — it isn't very pointy. A blunt comment is candid — it isn't very subtle. When we blunt things we make them less lively, sharp, or intense.
Blunt and blunder ("make a stupid mistake") may have the same origins — the Old Norse blundra, meaning "to shut one's eyes." So don't be blunt ("dull" or "obtuse") — here are a few things you probably don't want to do: 1) use a blunt ("dull") knife to cut your tomatoes; 2) make a blunt ("outspoken") comment that might hurt someone's feelings; or 3) blunt ("deaden") the sound of your favorite music by sticking cotton balls in your ears.