Note the 'black trails' are just shadows of the trails being cast on previous muck. Apparent trails in front of the airplanes are caused by the sun being almost directly behind the spraying airplane, the trail making a 'shadow tunnel' cast on and past the airplane.
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I do not have my own video evidence but I have see an aircraft lay down one of these dark trails and then a few minutes later another aircraft would lay down one of the persistent vapor trails. A shadow may account for one on a rare occasion but they simply are not all from shadows.