This looks like winter burn to me based on:
- absence of circular necrotic spots on the leaves
- boxwood blight (Cylindrocladium buxicola (syn C. pseudonaviculatum))should be less active during winter months as it is usually drier and colder
- absence of dark or blight on the stems
From here winter burn is described as:
Winter damage occurs on boxwoods (Buxus sp.) when unseasonably warm
winter days are followed by freezing temperatures. During the warm
period, the plant begins to come out of dormancy and the cambium or
conductive tissue begins to fill with additional water. When the
temperature drops below freezing, the water freezes. The expanding ice
splits the tissue resulting in death of the affected tissue. Cold, dry
winds pull moisture from the affected branch resulting in a
freeze-dried, "freezer burn" effect. Depending upon conditions, whole
plants, just sections of a plant or only the tips of branches may be
Boxwood blight is described in detail here.
- It is considered to be endemic in Europe and other areas of the world.
- Warm humid weather with abundant leaf litter to act as a source of infection promote the growth of the disease
- it can attack cuttings, containerized plants or well established plants
- fungal controls only give some relief
As is typical with many fungal diseases keeping the ground clear of debris, using a mulch to reduce soil splashing on leaves and good cultural practices will go some ways to preventing this.