Instructions
Set up the game so you're playing in Prestige mode. The only way to get to this mode is to level all the way up to 70, and then when prompted, choose Prestige mode. You will have to start over again at level 1.
Use the knife to make the majority of your kills as you earn higher levels. Knife melee kills are made when pressing the right stick down until it clicks.
Increase the number of melee knife kills to 100. At this point, you will have earned the title of "Blade Veteran." If you can increase the number of knife melee kills to 250, then you will earn the title of "Blade Master."