Help Detecting Dark Patch Among Other Dark Features

I’m looking for some help detecting a round, dark patch in the lower left of the attached image, and similar features in other images that will be in different locations. If the image was more uniform, it seems like thresholding would work, but unfortunately there’s ripples and vignetting in the image that are as dark as the feature I’m trying to identify. I’m not too experienced with this kind of stuff, so some assistance would be appreciated. The more specific, the better. Thanks.