Probably Reference Band.
Right Click on the 4Yr Grad Axis and Remove Reference Line
That did it. There were actually two reference lines. When I removed all reference lines my black lines at the end of the bars disappeared too. I didn't want that. But there was a second reference line that showed the 60th and 80th percentile of the 4yr Grad Rate. I removed that one, and the mystery bars were gone.
Thank you Michel!