I have some pictures with a nearly black background( RGB= 10-50). As I want to get it really black, I select the object, invert the selection and apply a fill layer rgb=0. In the newest version of Photoshop CC that does not work, the values are not changing and you see also that it is not completely black. In my older version CS5 I have no problems. What has changed?
I think you might need to show us a screen shot of your entire workspace showing the image, and the layers panels with the copied layer selected. Including the History panel might help as well.
It is super easy to do. Just copy to clipboard, and paste to this thread with Ctrl v (Cmd v)
Is your Photoshop document an RGB or CMYK document?