How do I block ads on YouTube in all WebKit based browsers?
You won't be able to block ads in the Youtube APP as long as you don't want to root your device. Blocking these ads requires changes to the DNS settings and you can't change these settings without root, nor can an app. 

When you don't want to root your device the only option I see is just to use Firefox (or a similar Browser), install the Adblocker addon and watch your videos through the browser. Less comfort, but the only option remaining without root.