I understand where you're coming from, but "open source" does not mean "I can read the source". If that were the definition, then Windows 2000[1] is open source.
That qualifies as Open Source under any reasonable definition of the word [1]. So being Open Source is no argument for or against either Adblock Plus or ublock origin, because both are.
Then you have a gazillion of "Open source licenses", that can be less "free" to more "free", like BSD, GNU, MIT, Apache, the free source license, ...