How to play all songs by genre in iOS 8.4 Music?
Found the answer! This feature is invisible and you have to know it's there.


  To shuffle songs within a genre in your library, do the following:
  
  
  Tap the "My Music" tab.
  Tap the selection in the divider at the top between "Recently Added" and the music list.  A drop down will appear where you can
  select a means to sort.  Select "Genre".  You can use this same method
  to shuffle within other sorting schemes (artists, albums, etc.)
  Scroll to your desired Genre.  Tap the IMAGE (or gray square) to the left of the Genre name, not the name itself.  A song within that
  genre should start to play.
  When you tap the currently playing song name at the bottom, the currently playing song will expand up.  Make sure the Shuffle icon is
  lit at the bottom. 
  THAT'S IT!  Enjoy!    If you have the "repeat" icon de-selected, playback will stop once all the songs in the selected genre have been
  played.
  


Source (apple forum): https://discussions.apple.com/thread/7106074?tstart=0