I set all the sliders for sharpening, NR, etc to zero. Other settings like brightness, highlights, etc were at the zero position.Here is a image of the sun reflecting off a gazing globe. Its a 100% crop, but no black ring.The effect is more pronounced the greater the contrast (steep brightness gradient) and smaller the blown image, i.e. stars and hot pixels are ideal to produce dark halos. The sun in your image is more extended and bright also outside the saturated region, so sharpening shouldn't produce as easily visible halos.This is a straight conversion from raw to jpg in LR 4, all the settings are nominal, no sharpening or NR.Isn't sharpening applied when using nominal settings? (I'm not too familiar with LR4, but other software apply it by default) If you push sharpening using unsharp mask I'm sure you can produce a dark halo around the bright dot in the right hemisphere. Just to illustrate the effect. (looking closer at your image there actually seems to be a dark edge to the white spots [not the sun] - implying some sharpening may have actually taken place?)
LR does not actually have a default sharpness, you can set it and save it for different ISO levels of your cameras as well as setup it to process images with pre determined colors, etc. However, you can turn all the settings bacxk to zero. Lightroom does not actually change the image, it just saves settings for each image in a database, and they can be zeroed or changed at any time. Then, when you render a image to jpeg or other format, the settings are burned in for that exported image, but the original is unchanged.
The white spots are deposits left by birds going to our feeded, and indeed might have some black or various colors in them, so I'd not try to judge them from the photo.
As noted, I did not have a blown out image from my 5D MK III, it does a very good job of setting the exposure, and we did not have many sunny days when I was testing it.