For what it's worth, I just recently bit the bullet and upgraded to iOS 8.4 from 7.1.2
7.1.2 PanGu jailbreak was getting the blue screen crash 0-3 times during startups until it actually loaded to the lock screen
In this past week with the TaiG 8.4 jailbreak, I have not noticed a single blue screen crash during a start up
Running an iPhone 5 though. I remember a few months back that there was an article mentioning Apple changing the flash chips they used for the 128GB models due to a problem with them
Maybe that's related to the blue screen you and others are experiencing?
Did you get your iPhone 6 soon after it was released to the public?