Hi Tash

Firstly, black spot is a fungus not a pest so I don't think that garlic water will work.

Here are the steps that you should take:
* remove all the affected leaves
* scrape back any dead leaves that are sitting on the ground under the plant and also scrape back the very top layer of soil (about 5mm) out to the drip line of the plant and replace it with new potting mix (this is where black spot spores hide)
* because black spot thrives in humidity clear out the branches and leaves in the middle of the bush - rose bushes whould have a shape like a drinking glass - with nothing in the middle for best results. You may think you are being savage taking branches off but it will thank you later on.
* give it good regular feeds - plants are much less prone to disease if they are really health - so a fortnightly feed with seaweed extract or fish emulsion would be great for them.
* also, give them a spray every fortnight with 2 teaspoons of bicarb soda in about 5 litres of water with a drop of dishwashing detergent and a teaspoon of seaweed extract. Spray on top and under the leaves until the leaves are literally dripping. You can get spray bottles at Bunnings or similar for a couple of dollars - you don't have to get a fancy pressurised one if you only have a couple fo roses.

Remember that any leaves you pull off and the soil that you remove can't go into the compost - it has to go into a plastic bag in the rubbish - of you will reintroduce it somewhere else in the garden.

Enjoy your roses - they should be blooming in the next couple of months