I found having all three sidebar options show at the same time confusing, even I would change the wrong one and wonder if I broke something when nothing would happen.
Now, only the option specific to the page you’re viewing is displayed.
So to change the Blog sidebar layout, navigate to your blog inside the Customizer 🙂
It should be working regardless though – the conditionals are core WordPress functions. If you’re viewing a page, the page sidebar layout will display etc..