I'm not going to participate in this retarded flame going on, but what I will say will probably shock a few. I think (and can back up) that Black Ops looks WAY better than Modern Warfare, and I actually enjoyed Black Ops more than Modern Warfare 2 or 3. But this is about visuals so....
First you have to look at what visuals mean. A lot of people think that the visuals of a game are just the technical graphics, and if the graphics are bad, the visuals are bad. This is not true whatsoever. The visuals of a game depends on TWO factors. The technical graphics, AND the visual aesthetic of the game.
The technical graphics of a game are what is running in the background to render what you see. It's the engine shaders, the models and collision detection, the textures, and so on. All those little technical issues. The aesthetic of a game is the art style in which the developers use in inspire their title. Visuals depends on these two things, and personally I feel that the aesthetic of a game is more important.
For example, Katamari Damacy has terrible GRAPHICS, but it's AESTHETIC is amazing, and therefore I feel that the VISUALS are great. See what I mean? Now lets talk about say... Call of Duty. In terms of technical graphics Black Ops, and Modern Warfare 2 (the game to truely compare Black Ops to in terms of graphics) are exactly alike, and hell Black Ops and MW3 are also incredibly alike too.
However, then there's the aesthetic of both the games, which is where Black Ops shines. Modern Warfare games have an obsession with the colors brown and grey, and it shows. The visuals are all sorts of bland and bleak, just due to how boring and grey it's visual aesthetic is. However Black Ops can't stop throwing enough colors at you. In Black Ops you are treated to a diverse, and amazing color palette and visual aesthetic that makes the game look so alive, and that's why I feel that Black Op's overall visuals are better than MW, and it's one of the reasons why I prefer it to MW. Now could Black Ops use a new engine? Possibly higher resolution textures or more complex shaders running in the background? Sure I guess, but it would barely do anything I assure you. I think the overall visuals of Black Ops, and all of Treyarch's games is great, and doesn't really NEED much improvement, but could use it.
Ehh... yeah. I also guess I'm back....