This is a spinoff from T198273: Create an API module to allow apps to read local overrides . Preferences email-blacklist and echo-notifications-blacklist have hardcoded transformations applied to them. As a result, meta=userinfo&uiprop=options API unexpectedly returns an array for these preferences, while interaction between core, Echo and GlobalPreferences becomes much messier.
The current plan I'm working on is to make the internal representation the same as stored by implementing a concept of filters for preference values between form fields and the rest of MediaWiki.