The Xbox Music team have released an update to the Windows Phone version of the Xbox Music fixes issues causing problems with the app. Version 2.6.666.0 fixes issues when syncing music from the cloud and fixes the issue where the Now Playing screen would just be a black screen.
Via Xbox User Voice:
A new release is available today for the Xbox Music app on Windows Phone 8.1. This release (2.6.666.0) addresses a number of customer issues including:
The black screen some fans were seeing instead of Now Playing
Several issues that were causing failures when syncing music from the cloud