Freddy,
It is nigh on impossible to patch up a rough textured surface without it being noticeable, but with a bit of care it can be made to look at least, presentable. The major obstacle is to remove the blown area, without making it spread. The best way, is to go out to where the pebbledash is solid, make a cut with a small disc cutter around the area and then working carefully, break out the blown area. It wont spread beyond the cut. Sometimes, you can remove a blown area back to solid area, by using a small chisel and again, carefully tap away at it. Try both ways, but NEVER "smash" at it, you will do more harm than good. Once you have the blown area removed,( and in the worst case scenario this could be right back to the brick/blockwork), give the area a wet down and scratch coat out to around 2 thirds of the thickness of the the original roughcast. Don't overlap onto the original. The mix for the scratch coat would be 4 to 1 plastering sand and cement with a waterproofer/plasticiser added to the water at the correct ratio. The mix for the top coat would be 5 to 1. Once the scratch coat has set and cured,( a day or two), mix up the top coat, wet the area again,and apply the top coat, bringing it out flush to the surrounding roughcast, paying attention around the edges,and with a harling trowel, cast the chips onto the wall, starting from the top, and working downwards, again, paying particular attention to the edges. Don't fiddle, and don't overlap. Because you are painting the wall, colours of the chips and mortar wont matter, but make sure the size of the chips are the same. This is a very difficult job you are taking on here Freddy, and if you have never done it before, it would be useful if you had someone come round to look at the job and advise you on the best way foreward.
Roughcaster.
