I was snapping some pics of hole-punch clouds in this cloud layer when I suddenly noticed this quite dramatic black contrail. Now the fact that it is black isn't incredibly surprising when you notice that there are black clouds in the sky. And the areas where the contrail is darkest are also the areas where the clouds are darkest.
But it was still quite interesting. It's actually quite a nice day, but there's a layer of cloud the west. The Eastern sky is blue with patchy clouds. This is a panorama looking south (showing an unrelated contrail that has developed into a fallstreak). Note the chimneys on the right.
I zoomed in on the dark trail:
The flight is Delta 2375 from San Francisco, passing over around 21:47 UTC
It was heading north west, and climbing through 25,000 feet. The contrail is probably the aerodynamic type, and not an exhaust contrail. The contrails became more sporadic as it got overhead.
Just behind that though was Southwest 542, on a similar path, but seemingly slightly lower. It also left a "black" contrail.
But this time it kept going longer, allowing me to see it was indeed an aerodynamic contrail:
This was more apparent as it went into the clearer regions of sky. Notice the contrails do not line up with the engines.
This enhanced shot shows the internal structure of the aerodynamic contrail
This is about the same section a bit later:
Compare with a contrast enhanced section of the same contrail in the backlit lighting conditions. There's no real significant difference however the less dense edges of the contrail are harder to see as they blend in with the clouds.