The blackening of glass can be caused by several issues. One of the main problems is the use of poor quality fuel and wood that has a moisture content of more than 20%.
Ask your installer to check the flue draught of the appliance as a low draught will also not help. As well as looking at the use of the air controls (within your instruction manual) to ensure they are being used correctly.
If the appliance is operated at low temperatures then you are not giving the air wash a chance to clear the particles from the glass. We would suggest having the air wash controls fully open and running the stove at a high output for short periods.
Use a cleaner, such as Hunter Stoves Glass Cleaner which is available from our website, or ceramic hob cleaner. This will lift the particles from the glass without causing the surface of the glass to scratch.