Box blight is caused by one of two fungal infections for which no treatment is available to gardeners. When you buy new plants, you should keep them in quarantine, away from other box plants, for about 3 months so you can check for infection before plantig it. Make sure you buy from a reputable supplier who will guarantee and refund or replace any affected plants.
Alternatives are things like lonicera nitida which has green and gold forms. Dwarf conifers can also be grown as a low hedge if you keep them regularly trimmed so they thicken up well. You could also consider step over apple trees which will be decorative as well as producing fruit but won't give such a solid effect.