This item was designed to counter tanks, but it seems to benefit tanks more than AP carries.
In every game I played, tanks build Blackfire Torch and then stack all tank items. Even bruisers like Skarnar do this. Since tanks usually have some form of sustained magic damage (even if it does little damage), they're able to continually deal health-percentage damage over time. That combined with the health, mana, and Kage's Lucky Pick, it gives a tank almost everything they need to do damage while being hard to kill.
When I use it, it doesn't seem that effective. Not sure.