They need you to watch their programming to make money. I doubt it's in their best interest to block local reception.
My antenna is indoors on top of the cabinet that holds my TV. I tried an outside antenna, but couldn't get any more channels than I could with the indoor one. For all I know about antennas, I knew very little about digital TV reception, as the change happened while I was still on Comcast. What I didn't know was to determine whether the antenna change was better, I had to rescan the channels each time. Since I found that out I haven't bothered to try the outside antenna again.
Most of the commercially available indoor antennas look like a computer tablet with a cable attached to the back of the set. I tried one of them, but it wasn't any better than my homemade one, so I returned it. If you get an indoor antenna, mount it as high as feasible, and turn the flat side of it toward the station's broadcasting tower, for best signal. Here, all the towers are on the tallest mountain, so one direction is adequate.