Well I don't think that it's possible to replace the menu background with another image. But if you just want a BLU version for a desktop background or something thats doable. The background images are located in "team fortress 2 materials.gcf", \tf\materials\console\.
I extracted the images and edited the colors to make a BLU version. Both the edit and the original are attached. If there is interest I could also make BLU edit of the second menu background, the Spy/Heavy/Pyro/Demo pic thats in the loading screens. The game also provides alternate widescreen versions of these files so I could edit those as well.