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.
https://extension.umd.edu/hgic/topics/macrophoma-leaf-spot 
http://extension.umd.edu/hgic/topics/volutella-blight-boxwood

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/...

Christa