I have that problem where i wanted to pick color from web browser in that case Google Chrome and when color picker was on any site it returned black color. So after searching the web for answer i remeber that chrome have Hardware acceleration checkbox. So there was the solution i found for my case hope to help someone.
So to recap. If you are unable to sample color from web pages using the standart method > chosing Eyedropper tool > clicking and dragging or CAPS lock and draggin to desired color on the webpage and it returned Black just go to your browser settings and try to disable Hardware acceleration. Have a nice day. 🙂