Lastly. click the box next to the song or artist. Blocking a song or artist will block them throughout your account in all playlists. Unchecking removes the block.
Using the Blocked Songs Tab
You can see all blocked songs/artists in one place, on the Blocked Songs Tab. To unblock, just navigate to Songs > Blocked in the left menu. In the Actions column click the unblock icon.
You can also block songs or artists in your Player Log Details. Use the filter to show only song play events. Uncheck removes the block.