Like other commenters I haven't found any
Blue Iris settings that really solve this problem but I have had some success by choosing the camera mounting location. I have one camera on the front of the house pointing straight out and it is always triggering when there is fog, mist or light rain blowing in front of it.
I have another identical camera that is mounted under the eaves and pointing somewhat parallel to the eaves overhang in the same general area as the first camera. The camera pointing somewhat parallel under the overhang is pretty much unaffected by fog, mist and rain. It has 10 or so feet in front of it that is under the eaves before the field of vision reaches beyond the overhang.
I think if it is possible to find some way to protect the first 10 feet or so in front of the camera it greatly reduces the reflection off of the fog, mist and rain. I realize that such positioning is not usually available or practical but it does seem to make a difference.