"This knife is duller."
Translation:Pisau ini lebih tumpul.
'dull' is used for people or situations'
This isn't true. Dull does have several meanings, like unintelligent and boring, as you say, but it also means not having a sharp edge. Check the Websters dictionary:
2: lacking sharpness of edge or point
a dull knife
'blunt' should be used for knives (in this case)
I disagree. Dull works better, I think. Dull implies that the tool is supposed to be sharp, but has gradually worn down and become dull. Blunt, on the other hand, sounds like it is intentionally made so, not just from typical use. For instance, knives become dull from use and must be sharpened occasionally, but if you want to practice knife fighting, you would blunt them, or use blunt knives.