Richard,
Scalemaster is a good product for the do it yourselfer.
We typically use a special descaling acid, which we circulate through the heating coils using an acid pump driven by a 2 HP electric motor.
Usually black nasty looking stuf coming out of the heating coils is not a sign of scale build up, but of something else, check your inlet pressure between pump and coil, and then the outlet pressure between the coil outlet and hose / hose reel, if the pressure fifference between the 2 places exceeds 250 I would descale.
The black stuff coming out of the gun, could actually be several things, the first that comes to mind is that the inner tube of your hose is detiorating due to overheating?
Has the burner ever stayed on whil trigger was closed?
What other than water do you run through your heating coils?