Rarely get blackfly on my broad beans and last three years have seen almost nil.
I plant marigolds right next to where they grow in a raised pot, spray them when first setting about once a week with a water and soap spray and when the flowers appear remove the tops.
Being on the coast, the old boys always tell me that Blackfly tend to be more prevalent inland
My Dad about 8 miles north of me and inland, does always get a overnight heavy attack of blackfly. He uses a water and soap spray and that stalls them. That does the trick with again a weekly dose of spray. He also pinches out the tops.
Do know some others that grow nasturtium close by, as the blackfly love them and are attracted to them.
But, you have to cut off the affected leaves as they keep coming back and could spread if not dealt with.
Being the spreading plant it is, they tend to grow in controlled buckets.