This is the front. I'd like to underline that some of the fungus/whatever it is on the backside is yellow, if you can't see from the photo. Many thanks :)

You have rust on your rose

Here is a link that might be useful: Rose Rust

Uhh thanks!!! I was thinking it might be rust but I thought it should be primarily yellow, not blackish...I guess the black is just oxidised yellow.

Yes, it's rust.

No, the black isn't oxidized yellow. In fact, the black is even worst. If you had a microscope, you'd see that the black are larger spores (when compared in size to the yellow spores also on the underside of leaves.) The black spores look like two wizard's hats joined at the base. What makes the black spores worse is that those are where the disease overwinters and will be the source of the disease next year.