Broad Beans. Black Spots.

I've had a really good crop of broad beans (Dobies. The Sutton)

When I shell the beans,  there is a small black circular spot on the skin of some of the shelled beans. This spot is on the thin outer skin of the the bean, there is no mark on the inner green bean itself.

The spot is between 1 and 2 mm across.

Can anyone tell me what is causing this?