Well i never blanch peas or broad beans done it for a few years now,and both taste great.Mine never seem to lose flavour and take very little cooking,i just boil the kettle pour just enough boiling water to cover,bit of salt and sugar bring back to the boil simmer for 3 mins,drain then return to the pan to steam off then plate them up.As for how long can they be left on the plant i pick all that i think are just ready pod asap then put them in the freezer or use on the day.Just to add this is my way and it works for me others may not agree,i also freeze and store my peas in a large tupperware type tub.