If your pipe is constantly getting coated in black soot than most likely this means you touching the flame directly to the pipe. So yes, most likely you are applying way too much heat.
You want the pipe to be a bit above the tip of the flame, the tip of the flame not actually touching the glass. It will take longer for the drugs to start to vaporize this way but this is a good thing. You want to go slow and steady. Once it starts vaporizing you can control the rate by lowering and raising the distance as well as by rotating the pipe around.