I looked down at the cloud layer and saw what looked like a very long black rectangle in the cloud layer

While on a flight between DFW and Tallahassee (TLH) probably an hour into the flight, I was looking out the window at the cloud layer below and saw in the south a very stark black, long rectangle in the distance for about 30-40 seconds. I kept trying to look more closely, but I can share that it had sharp lines and looked long enough that maybe it included multiple rectangles, but was not very wide. But it was black and looked odd against the clouds. It did not seem to be moving. Then after after staring at it for around 30 seconds it seemed to disappear - possible in the clouds?