I havent seen this recently, but that sort of momentary blanking can be caused by static electricity buildup on an improperly or ungrounded dish dishcharging. It can also be a ground loop problem inside the house not involving the dish.
There also was a problem a little while back with the type of mpeg4 transcoding equipment that directv uses improperly handling certain block boundaries, which led to occasional picture breakup and the infamous zipper/brippp sound. I havent seen that problem very much lately, but I have seen a few blank or repeated frames on occasion and perhaps at about the same rate. So maybe their 'fix' for the zipper problem was to give you a few silent black frames instead of an annoying little sound.
This problem depended on certain channels and the mpeg2 gear they used, in conjunction with certain models of the mpeg4 transcoding equipment that directv uses. Its 110% a directv issue, and the problem doesnt manifest itself in purely mpeg2 transmission setups like local broadcast and copper cable networks. I dont know if this occurs with Dish, I dont follow them at all.