What is the best way to earn blue energy?

Besides the free 50-blue-energy gift from the June's Journey email newsletter, extra blue energy can only purchased with diamonds. You can also spend diamonds to get the increased capacity bar which will still accumulate energy at the same rate of 1 energy every 2 minutes, but will hold 90 instead of 50 energy, so you can be away from the game for 180 minutes (3hr) instead of resuming in 100 minutes. When you buy it, your capacity fills and you get 90 bonus energy on top of than. But the extended bar is only good for one Secrets events; you'll have to buy again the next Secrets event. More info at https://wooga.helpshift.com/hc/en/27-june-s-journey/faq/3249-what-is-secret-energy-and-how-do-i-earn-it/