1) The viewfinder won't suddenly be black if you use both a CPL and your glasses, regardless of the CPL rotation angle. (The glasses are linearly polarised, the CPL circularly.)
2) There is a slight additive effect in that you will be seeing only the component of the circularly polarised light passing through the viewfinder that is also polarised in the plane of your glasses. You'll see a mildly exaggerated polarisation effect on reflections and saturation, and a slight decrease in brightness.
3) You may see some "rainbow" artifacts through the viewfinder. I'm not sure of the source of these. They may derive from stresses in the plastic lenses of the glasses (in my case - yours may be glass and the same thing might apply), or in the focus screen (my best guess), or some issue with internal reflections. These are a minor annoyance.
4) Wearing the glasses gives you a great preview of what the scene will look like with a CPL, but as noted, if you are not using a CPL, this may be misleading.