The skin I'm using is a third party skin called BetterAreoVistaX, but I've tested with some of the skins that came with the program and they don't provide the full transparency either.  The slider I'm referring to is the one found when you click "Modify style" next to the skin one chooses, and then you click "Transparency", the option in the middle that lets you set the transparency on either the menu, taskbar, the top bar of a browser window, or the left click menu. There are sliders under each of these that lets you decide how transparent you want them to be. Sliding all the way to the left gives full transparency (from what I gather) while sliding to the right makes it more blurry. 

My Windows version is Version 23H2 and it's a 64-bit system. The version of WindowBlinds I've installed is 11.02.