In my opinion the easiest way to learn it is -> learning by doing or with a tutor
Just open a blueprint and try to figure out how everything works. Of course this will take a long time, but after some time you will know how everything works. When you have any problems, you can always post a question into the forum -> we are always here to help 
Some good ressources for blueprints:
https://docs.unrealengine/latest/INT/Engine/Blueprints/index.html
https://wiki.unrealengine/Main_Page
https://wiki.unrealengine/Videos
Also make sure to search for specific stuff. e.g how to create a projectile -> then you will find plenty of information on the internet (youtube, forum, ,…)