It looks like your boxwood is showing symptoms both of Macrophoma Leaf Spot and Volutella. Information on both of these diseases is provided in the links below.
Boxwoods can be susceptible to these fungal diseases when they are stressed by environmental conditions -- e.g., too much moisture, root competition, etc. You can prune out the affected areas. You may want to do the pruning gradually over a few seasons to reshape the plants. Pruning helps to increase air circulation and reduces chances for fungal infection. Take a look at our video about boxwood pruning. https://youtu.be/3i8mcoKapeY
Refer to our publication on boxwood for additional information. http://extension.umd.edu/sites/extension.umd.edu/files/_docs/programs/hgic/HGIC_Pubs/TreesandShrubs/...