Hey there! I too had the same problem. Hopefully i found a solution that worked for me. Install spotify on your computer and play the song that you blocked mistakenly. Then open spotify in your mobile. You will now see the song that is playing in your computer if both your mobile and computer is connected through same account and wifi. There you can unblock the song by pressing the block icon again(it would have turned red)
Open Spotify on your mobile device; show "Your Likes". Open by pressing the icon in the upper left coner. It looks like a person. This will show all the songs that you've liked, plus the songs that you've blocked. Unblock the song(s).
How do you view your "Hidden" playlist? I remember it being there when I had the free version of Spotify but since I got Premium it's not there anymore. It also seems they changed the system from block/ favourite to like/dislike. Not sure if this is a distinction between Free and Premium or a recent updated.