The Ever Black Shadow is located inside Mike Toreno's Ranch. Appearing 24/7, the shadow is jet black and can be found taking up a large portion of the Ranch. The source of the shadow is unknown, but could possibly be an intentionally placed texture. Many people in the GTA Myths community have theorized that the shadow acts like a "censor," and is covering up something secret, such as a clue for a myth.
The shadow does cover up a framed photo of the Big Pointy Building in San Fierro, and a few illegible pieces of paper, so this theory does have some grounding in reality. Within the game's files, there is a texture called "mp_turenoshadAa" that appears as a solid black texture that matches the shape of the shadow. It means that the shadow is intentionally placed, but it is unknown why Rockstar would do such a thing.