Why does my truck release black smoke sometimes when I get on the the pedal and then other times, when I seem to be applying the same amount of pressure, there is none?

As you know, black smoke is the result of unburned fuel in the combustion process. Sometimes your truck will blow smoke when it is accelerating under load, or sometimes under hard acceleration if you haven't romped on it in a while. If the latter, it's usually just clearing accumulated soot out. It will also tend to smoke when you accelerate hard in low boost situations (until the turbo spools up).

also, if it is cold outside, the air is more dense and thus you get a better burn of the fuel and don't have as much unburned fuel to come out as black smoke.