As far as Blink ... it is open source, that means any changes Opera make to Blink itself must be available to anyone. (Open source does not mean public domain.) Apple didn't originate webkit - it was based on the (also open source) khtml engine used in the Konqueror browser on Linux and FreeBSD. So any change Opera, Google, Apple, Konqueror or anyone else make is available to everyone ... if they can figure out how to use it. Truth is, khtml is quite far behind webkit because it is hard for them to incorporate all the updates into their code.
Now, Opera's UI is totally theirs, Google has some proprietary stuff of their own in Chrome, likewise Apple in Safari though Konqueror is completely open source. But not one of those entities sells the browser, so ...