I blanche and freeze them when they are still small and tender, definitely before the scar at the top of each bean turns black, and they stay green in the freezer. I have missed quite a lot this year, but no point in freezing them now as I know they will be too big. I did once try skinning them ie removing the thick outer skin, then making the inner green bits into bean burgers - it took forever and I decided that life is just too short.