Hi there, am a London professional gardener for private gardens and this is what I have learnt which has helped reduce buxus blight by 98%. Most buxus plants we buy over here are treated whilst growing at source of suppliers with fungicides. Then when we buy and put into our gardens where they are then untreated, unknown to us, they are carrying the spores anyway.
I use two types of fungicide and also Topbuxus fertiliser. I keep them fairy dry and only water at the roots. I cut out immediately any diseased dying areas and remove debris. I do not use a blower, but carefully take away all dead material inside and around the plants. I wash all my cutting tools and hands in a bucket of diluted bleach with water from plant to plant or area to area to kill off any spores.
The bleach mix is corrosive, so oil has to be wiped on cutting tools. Olive oil is also good as a cleaner and lubricant on tools. If wearing gloves, use plastic ones so they can be dipped in a mild solution of bleach.
Hope this helps.