I have been using 1blocker and I think it used to block them, but now it does not.
Previously, previously, previously, previously, previously.
If there's a functional way to do this that doesn't involve routing all traffic from the phone through a proxy that you control (mostly simply through the "VPN" configuration), I don't know what it is. I would certainly also be interested in that, though.
(I tried the proxy thing for a bit. I think it's more painful than it's worth. But I also just don't watch Youtube videos on my phone.)
The official ad blockers just block requests in Safari. They don't block YouTube ads because you're viewing YouTube in one of two ways: the app, which isn't Safari so blockers aren't even loaded. Or through Safari, where they have no way of determining if the video you're loading is legitimate or not, and don't have any control over the media player.
At least in Safari you can usually just swipe the timeline needle to the end.
i used to use 'mikoto' from the cydia package manager, but I have since simply stopped watching videos on my phone since I needed to update the os (the process of rooting is a pain in the ass, my time has better value, etc.).
Weblock seems to block youtube ads. You get a few seconds delay when it tries to load an ad, then the video plays.
Sadly weblock uses IOS's advanced wifi settings, so it does not work for cellular data. Happily it works for all apps on the system, and not just Safari.
I don't think any of them do anymore. I know that using iCab Mobile with the easylist Adblock list ey are blocked, but only within that one browser.