This knife is blunt. This is correct, there is no need for the word more.
Pisau ini tumpul. = This knife is blunt.
Pisau ini lebih tumpul. =This knife is more blunt.
Should also be able to say 'this knife is more blunt'
Yes agree blunt is probably a better word to use than dull.
Isn't it blunter?
I have never described a knife as blunt. Is this trying to say the opposite of this knife is sharp? If so in the midwest we would totally say this knife is dull. As an English speaker I do not understand what concept we are trying to communicate.
Not a phrase we would use in English. "This knife is blunter" would be better