Black line shaped craft keeping same speed as plane then closed in, changed angle when got closer and reflected sunlight

I was a passenger on Southwest flight Nashville to Boston, about 45 min to 1 hour into the flight. I spotted off of the left/north side of the plane, an all-black, flat line shape with some depth, which was keeping the same speed as the plane. It was about 1/4 mile out, at the same altitude which was around 30k feet or above. I watched for 2 to 3 minutes trying to make out the form, then the shape started moving closer to the plane maintaining the same height. Similar to 2 cars on a highway, one moving from the fast lane to the middle, while you're in the right lane. When it got closer, it then changed angles, ducking down lower and when it did, the sun reflected off of the body like a metallic reflection. That was the only moment I saw it go from a black line to another shape which was maybe triangular? I told my husband when he picked me up at the airport and he thought maybe a stealth bomber. But that would have been much bigger, I doubt they would fly it towards a passenger jet, and I don't think it would have a metallic shine to it. This was also a flat line with no visible cockpit area. The weather was clear and dry.