Mystery Plant is Black Nightshade (Solanum nigrum)?

I have something growing wildly in one of my herb boxes that I'm not sure what it is. I have purchased quite a few different kinds of seeds from you that, to my knowledge, never made an appearance. I was living in a spot that got almost no light etc, so the lack of success is not likely anything to do with your seed.
But now I have moved and my boxes get lots of light and lots of sun and stuff has sprouted. Stuff I didn't plant this year.
This one plant in particular is about 6 - 8 inches high, and has green serrated leaves with a reddish purple tinge on many of the undersides. It had tiny white, almost trumpet-shaped flowers with large yellow thingies in the centre. The flowers turned into deep dark green "berries" which have now turned a dark eggplant colour.
Can you help with an identification here and let me know whether the "berries" are edible? Also let me know if the plant is edible at this late stage, and whether the "berries" should be saved for next year's seed.

This sounds suspiciously like black nightshade, Solanum nigrum. The seeds last a long time in the soil and only germinate when conditions are right. Birds spread the seeds far from the mother plant and it has become a very common weed.
According to Hortus Third, it is reported as poisonous, but cooked locally as potherb and the ripe berries are made into pies and preserves in local areas. I would not take the chance of eating it unless I was very hungry! If you do decide to eat it, make sure you cook it, because many alkaloids are denatured by cooking and this may be the clue to its possible edibility.