This looks to be "black flagging" of arborvitae. The specific reason for this condition is unknown but it rarely causes long-term damage to the health of the tree. My recommendation would be to wait until spring and then prune out the blackened branches. You may consider bringing the branches to your County Extension office to be submitted to the diagnosis lab - just to be sure.

Also, take a look at these publications for more information:
https://pnwhandbooks.org/plantdisease/host-disease/arborvitae-thuja-occidentalis-black-flagging

https://hyg.ipm.illinois.edu/article.php?id=15

Sincerely,
Kelly Jackson