Hard to say what that is. Doesn't look like herbicide to me. I think it's either environmental damage or a touch of grey mold. Have you had frequent rain? I would just prune and stay on top of spraying. Your plants look very healthy otherwise. Watch newer growth for tightly curled leaves or distorted leaves with frilly edges. Those are usually the signs of herbicide injury. Except Roundup/glyphosate, which first manifests as bleaching/yellowing at the bases of leaflets on the new foliage.

Done and done. I did find some damaged leaves on other plants in the bed, as well as one tomato that had a bad area on the underside (picture posted). While the rain in July has not been excessive, it has been extremely humid and cooler than normal, which I now understand facilitates the growth of grey mold. I do not see the signs of herbicide damage that you indicated. It's always something, isn't it? I'm looking at my best harvest ever if I can only keep them healthy for the next couple of months.

There's a "mushy" form of BER that can look like that. My Malakhitovaya Shkatulka last year were particularly prone to that particular form. It could also be a mold that got in through a blemish or crack in the catfacing. If it's just the one fruit, I wouldn't worry too much. Cool and humid is definitely weather that all kinds of molds love.

Knock wood, I've been extremely fortunate in regards to BER this year. On all my ripening tomatoes, I've only found BER on one other. Normally, my paste tomatoes have been prone to it. The only thing they seem prone to this year is EB, especially the Opalkas. The others are barely touched by it.