Hi Jacqui, I grow Borlotti and other beans for drying each year. I tend to leave them on the plant as long as possible, so when I harvest them they are fairly dry already. I pod them then leave the beans out on trays under cover for about a week. I then put them into plastic tubs (takes much less space than the trays!) in my kitchen without the lid on – and every now and then I give them a toss to brings the lower ones to the surface. I guess I leave them like this for about 2 weeks (maybe longer as I tend to forget about them) before closing the lids. I’ve never had any go mouldy.
I live in France and something I have learnt from my neighbours is to store them semi-dry too – they are delicious and cook much faster. Harvest them when you have a good sized bean, pod them and then simply bag them up and freeze them. No need to further dry them, blanch or anything else.