I can't see how to enable it, but does it help my issue?
You don't enable it, you just don't check the "Force blur effect" in WBConfig. If the skin is made without taskbar blur in SKS (e.g., Lantana), you get non-blurred transparency now as you should. If the skin is made with taskbar blur enabled in SKS, you have to go into SKS and change its setting, you can't (and I don't think ever been able to) turn off blur in WBConfig.
If you look at Lantana now without the "Force blur effect" box checked it should no longer have the color block at the left end of the taskbar (and the rest of the taskbar has the appropriate transparency). I believe the same will be true of the skin you showed in your first post.
On a taskbar with blur enabled, fully transparent areas on taskbars such as at the ends or rounded corners have had the blur applied to those transparent areas for a very long time, I want to say "forever" but probably not. and that does look poor to me. Best not to have blur enabled on those.
So anyway now you should see this: