Rose blackspot is caused by a fungus which causes purplish spots to appear on the leaves which can then turn the leaf yellow before it drops off altogether.  It thrives in wet weather which causes the fungal spores to splash from one leaf to another, and even when it's not raining, the wind can blow the spores around so its very hard to avoid black spot in the UK. 

The fungus doesn't kill the rose, but it does spoil the look of the leaves and can weaken the rose, reducing the number of flowers it produces. 

Though impossible to prevent or destroy completely, you can reduce the impact of blackspot by: 

• Choosing disease resistant roses
• Picking off the infected leaves
• Use a fungicide rose spray from early on in the season and throughout the season to help protect the plant
• Collect all fallen leaves and burn or add to your council bin rather than compost locally as the spores can overwinter in compost. 
• Prune back any badly infected parts of the rose and destroy the pruned stems.
• Feed the rose well through the growing season to encourage strong growth and assist resistance to disease. 

As the fungus mutates regularly, becoming resistant to fungicides, it's almost impossible to get the better of blackspot, so best to contain it as much as you can and ensure your roses are otherwise healthy and strong, so you can enjoy the flowers and overlook a few blemishes on the leaves. .