There is no mistaking the signs of this disease.
It takes its name from the fact that it starts in the blossom end of the tomato fruit - in other words the end which is furthest from the stalk / calyx.
Not a pretty sight, is it? And there is nothing you can do once the fruit is affected. It will spread throughout the fruit until the whole thing goes brown and mushy.
As if that weren't enough, look at this:-
What a sorry sight! That is a Purple Sprouting Broccoli plant that has been attacked by the Cabbage Root Fly. The fly's maggots eat into the plant's roots, and gradually degrade its ability to take up moisture, thus causing it to collapse. Even before this happens, you will see the plant's leaves fade and take on a matt finish, at which point you know the plant is doomed.
This is what a healthy PSB plant is supposed to look like:
Unfortunately even my spare plants have been affected, so as a last resort I have sown some more PSB seeds. They probably won't mature in time, but I feel obliged to try. This gardening malarkey can be a bit depressing at times...
Looking on the bright side though, I keep reminding myself that this year the Radishes were brilliant; the Broad Beans were brilliant; the Potatoes were (are) brilliant; the Lettuces were (are) brilliant, etc, but it's maybe not reasonable to expect EVERYTHING to be perfect. I must expect a few failures, or at very least a few casualties along the way.