Call of Duty Black Ops for the Nintendo DS beats all of the other Call of Duty games on the DS by over 9000 times. A real treat.
Graphics: 10
This game's graphics is very nice and a real visual treat. I would have expected this game to have mediocre graphics because of how limited the DS could handle 3D graphics but the developers really did push the limit of the DS. And what ended up was probably the best FPS on the DS. For instance, the people in this game actually look like real men. You can see their faces clearly. When a grenade goes boom, the explosion effects are really realistic. Compared to World at War on the DS, the people look like polygons. Their faces are flat and blurry. The explosion effects from a grenade are very pixelated. I wish I had a picture to show you the comparison. The only downside is that it can lag a lot sometimes during heavy combat, and in rare cases, crash your game and your DS.
Sound and Music: 9
The sound effects are pretty normal. It's what you would hear when you fire a real gun. The death sounds are identical to World at War, and the different languages that enemies speak as they fight make sense. For example, when you are breaching a Cuban church, you can hear someone saying, "Vamos! Vamos! Vamos!" which means, "Go! Go! Go!"
The music is where this game's sound really shines. The soundtrack is very epic and catchy; such as when you are escaping Afghanistan in a truck and a helicopter tries to blow you up. The string instruments play an uneasy song while the drums are so fast that it makes your heart beat faster and faster until it's finaly over.
Addictiveness and Depth: 8
This game can get quite addictive because of how fun the missions are. There are a total of 15 missions with each one being very unique. You could be escaping a prison in Mission 2, or flying a fighter jet in Mission 12, or assassinating a foe in Mission 7. Apart from the missions you've got a campaign mode, an arcade mode (with points and a timer), training, zombie mode, multiplayer, the killhouse (just like training, only with more guns), and if you input the right code into the pro action replay, you can access test levels used by the developers. 
Difficulty: May Vary

The difficulty will vary depending on what kind of gamer you are. If you are a casual type, you may even find the easiest difficulty harder than a nut. If you are a hardcore type, this game will be a breeze. Even on the hardest difficulty. I am a very casual gamer, so I give this a 9. On top of that, there are three difficulty options: Recruit (Easy), Regular (Normal), and Hardened (Hard). It will also depend on how you like the controls. You use the D-pad to move your character in eight directions and you either use the buttons or the touchscreen to rotate and change your heading. Hit the shoulder buttons to fire your weapon. Some people hate it, some people like it, I think it's OK. I feel that it's better than the Playstation's controls.
What do I say?

I'd say that this game is better than all of the other Call of Duty games. It's got better graphics, good music, lots of game modes and more. I would give it an 8.9/10.

I've tried some of those games before and the FPSs on the DS are not always that great but I guess this one was pretty good seeing the rating you gave it. I haven't played this one myself though.