Hi Mark,
this is my first post here so excuse me if your problem has already been dealt with. As it happens I have just blanched and frozen about 5lbs of French and runner beans as we've been eating them with every meal for a while, My beans were, in the early part of the season attacked very heavily with blackfly, apart from rubbing them off with my fingers I have found that collecting ladybirds and putting them in the bean bed has been a very successful control method. I won't use chemicals so it's natural or failure for me. My daughter has taken a very keen interest in what is the gardeners friend and informs me that a ladybird will eat 50 aphids a day so 20 ladybirds is a serious challenge to the aphid population.
Good luck with it and make a daily check on the blackfly population between yours and the ladybirds efforts you should be able to keep them sufficiently at bay.
Jake