MW2 Stealth Tag Unlock: How to Get the Art in Modern Warfare 2

To unlock the Stealth Tag art in Call of Duty: Modern Warfare 2, you need to complete the following challenge:

Hidden Threat: Reach level 8 with the Combat Knife.

Once you reach level 8 with the Combat Knife, the Stealth Tag art will be unlocked and available for use in Gunsmith customization for applicable weapons.