from the general appearance it's clearly a member of the Solanaceae. I can't open the photo to give me anything but a distorted view but the shape of the corolla - rather flared makes me wonder if it might be a Physalis rather than a Solanum. The commonly found Physalises, like the Tomatillo (P. ixocarpa) tend to have yellower flowers though.
I too was thinking it was the black nightshade but I wasn't 100% sure. My son found the plant growing in the field his horse grazes in, and had asked me what it was. I have instructed him to return to its location and take another clearer picture, then I can see what fruit has developed on it since his last visit.