Getting rid is difficult, control is the thing to aim for. Mix one part COW's milk to two parts water in a spray, and spray with that till run off. Has to be cows milk, any other kind of milk won't work (almond, coconut, goat's). Repeat the treatment weekly. Alternatively, seek out a fungicide spray which treats for this condition.

If any of the leaves are really badly affected already, pick those off, and do not allow any fallen leaves to remain on the ground - remove them as soon as they drop.

......also I would add to make sure the rose plant is not under stress at all. Roses are both hungry and thirsty plants so make sure you both feed and water well. Any plant that is not stressed with grow and flower better.

I tried the milk solution 2 weeks ago after seeing the beginning of blackspot on a couple of lower leaves. So far, I haven't seen any more. I did remove the affected leaves as Bamboo has advised.