I use a chemical called Permethrin which was around in general horticultural use for decades until the EU decided to ban it along with just about every other pesticide and fungicide. It's non-toxic to birds and mammals (other than cats) and kills all insects so you should spray with it when the bees aren't around (eg late evening or after blossom). It's still available on Amazon (Lifesystems Permethrin) and needs to be diluted down 8:1, but it works very effectively on aphids like blackfly.

Greener alternatives are soft soap solution but that doesn't seem to work too well for me.

Hi, there isn't a lot you can do to prevent blackfly, all you can do is spray with insecticide when you get them, the sooner you spot them, the less timethey have to damage the leaves, Derek.

Thanks guys. As I have a cat so I will need to use the soapy water method. Do you just gently wipe them off with the soapy water? and I assume you have to keep on top of it regularly to avoid build up.

You just spray the solution onto the affected areas. No wiping involved. Some people just use a solution of washing-up liquid. But most good garden centres stock the horticultural soft soap stuff.

I think you are worrying unduly about my Permethrin option . There isn't a problem unless you spray it onto the cat or into its face. And you would be hardly likely to do that. I only mentioned it as a precaution.