A new patch has gone out for MTG Arena that aims to fix loading times and to reduce latency issues including the "black screen of death" experienced by many players. This will require a client restart to download the update.

Quote From MTG Arena Admin

  • Various fixes to improve loading times and reduce latency/network related issues (e.g. getting stuck on a black screen, persistent "waiting for the server" messaging, etc.)
  • Players should no longer inadvertently end up on the debug console.
  • Adjusted mouse detection/hitboxes on the Zendikar Rising battlefield to prevent situations where players could not target/mouse-over-zoom their opponent's lands and/or Planeswalkers.
  • The Nahiri's Stoneblades card sleeve now displays the correct artwork.
  • Corrected the artist credit for various Zendikar Rising showcase card styles.