Actually, "expanding" of light is very natural. When light flares as it bounces around the lens, it bleeds from areas of light into dark. (and that is why lens flare kills Dmax).
If you don't delete the darks before blurring, you will have to erase more later, and end up with erased (unblurred) areas along the border between skin and the darks... which itself can look unnatural.
For example, if the model has a beauty mark, and you don't delete it before blurring, you will have to erase the mark and the area around it where the dark beauty mark was "smeared" by the blur from the blurred layer, and that will leave a soft skin cheek, then a beauty mark with un-blurred skin right around it which is unnatural.
If you erase the mark first, you only have some skin blurred over the mark, and can erase that and leave all nicely blurred skin around it.
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