Pilot here, and that is another airplane. We see this daily. It's great that you pay so much attention to your surroundings and notice details like this, though. Nevertheless, the object is indeed another airplane that had crossed the path of yours underneath and has consequently been cleared to climb to a higher level. The shape of the contrails is nothing out of the ordinary and indicates a medium sized airliner and the change of shape is due to the high relative velocity. Mind you, you are on a diverging course with the other aircraft and you're flying with an approximate speed of 850 km/h / 530 mph. The lack of other visual references to hold on to makes it hard to identify it as what it is. Later on in the video the other airplane seems to have been cleared to proceed in the same direction as yours which is why it stopped getting smaller. Up in the air, airplanes need to maintain specific separations as safety margins to fly the same routes (yes, we actually do have 'highways' in the sky) and that is most likely the reason why the other airplane first crossed your path and then flew parallel to you. In case you cannot make any sense of what you are looking at AT ALL: the dense dark cloud are the contrails (water vapour + exhaust gases) as you look at the other airplane from behind. You're looking at its tail as it flys away from you which is why the contrails look so dense. Then, as you pass the other airplane the contrails stretch out as your perspective changes rapidly (due to your high speed and the close proximity of the other airplane). One thing is remarkable though: the contrails are rather dark on this airplane. Normally, they are white for modern airliners and sometimes can appear darker compared to the background when they are in the shadow of other clouds. This one is very smokey. Probably an older aircraft which have less efficient combustion which leads to black smoke.
As usual in the spook/ufo section no personal comment required from me.