I'd suspect the black spots are unrelated, most likely Rose Black Spot (Diplocarpon rosae), a fungal disease. Putting out old cat food is however not a good idea, likely to attract rats and other unwanted animals after some free food. Find someone with a pet which can use it in the intended manner.
As Michael F said, rose black spot is a fungal disease. This fungus's spores are ubiquitous - they tend to find it's way to roses anywhere. However, conditions required for it to take hold includes sufficient moisture and the right temperature. In your case, I think temperature condition is what explains the appearance of black spot in your roses this late. The fungus optimal temperature for disease development is 75F, or 24C. I usually start to see the spots once the temperatures go above 20C for any significant stretches. This year, the weather in the Pacific North West has been somewhat atypical, as it had been colder for longer. There were periods of warmth in April, but it's been downhill since then, with temperatures keeping mainly below 20C, until recently. That would explain why the blackspots had not shown up until now. There are no proven remedies for prevention or treatment of blackspots that are systemically absorbed via the root system. Therefore, your addition of cat food to the rose bushes is very unlikely to have any specific effect on the black spot. However, if you did manage to improve the health of the rose through the application of cat food, it helps to a degree - healthier plants are, in general, more resistant to black spots (and other fungal diseases, for that matter). Cat food won't last more than an hour in our garden - our dog will find it, dig it up, and excavate the roses at the same time.
