Blue electrical tape. I have used it for a few years (black tape before that). I have observed no change in point of impact--either at the range or in the field. In response to John Moses' comments, rain falls almost continuously for days at a time here during hunting season. Some days it doesn't rain a drop--but the mud is always there. Very few days when the ground is dry. Rather than add and remove tape, I just tape up for the season and call it good. Here's a few pics:
Anyway, I was skeptical about how tape might affect accuracy, but as mentioned, both on the range and in the field, I haven't noticed any changes in POI when using tape.