The latest beta of Start8 allows you to pick your own start button image, even if one is supplied by the Windowblinds skin in use. There should also be an option in Windowblinds titled 'Always use the OS start button rather than the theme one'. EDIT - also see the post above.
At present, you cannot wholly disable Start8 skinning (I personally would like such a feature as I like the default Win7 skin).