detect page class only

If I understand your question, you are saying that when showing a post, you would like to know which category or categories are assigned to the post? So, if WP247 Body Classes had classes something like:

body.is-post-category-unknown or body.is-post-category-17

would that meet your need?

These could actually conflict with posts named “Category Unknown” or “Category 17”. So I might need to use “postcategory” as the keyword instead of “post-category”. But, otherwise, I think this is doable and worth including if it meets your needs.