The picture below was taken by Flickr user Magic_lamp while on a Ryan Air fight from Portugal to the UK this May. He says, that despite experience taking this type of photo, he did not notice the dark spot in the upper left of the image along the curvature of the earth when he took the photograph.
At first glance, it appears to be a speck of something stuck to the window of the plane, but as you look closer it become apparent that what ever was there was not stuck to the plane. When you blow up the object things begin to get interesting…
As you can see, the object does not appear to be any sort of known aircraft or balloon. At this level of magnification we can begin to see that the object appears to be three dimensional. When you look at the cloud tops and the engine cowling, it appears that the object was being lit by the same light source as the engine and clouds. This becomes clearer as we further enlarge and enhance the image. This time to 1700% of the original size (this was done with SmillaEnlarger at the default setting).
Here we can see from the shadows on the object that it appears to have convex sides and that the side of the object facing away from the viewer may be curved or have curved corners. Additionally, we can see that the 4 blue dots along the dark side of the object that were visible at the lower magnification appear to be part of some sort of circular structures stretching the width of the object. Unfortunately with only one photograph we have no clue which direction (if any) the object is traveling in, but if we assume the object is traveling away from the camera, the dark section we are seeing may be the external portions of a propulsion system. This could provide a plausible explanation for the darker coloration which could either be the result of exhaust from the system, or the propulsion system being recessed into the craft to reduce the thermal or infrared signature of the craft (like was done on the Stealth Fighter).
What we are seeing in this picture is anybody's guess, but baring a well executed hoax, it appears to be some sort of UFO. Whether its origin is terrestrial or extraterrestrial, while I cannot personally vouch for the photographer and I am not an expert at image analysis, this appears to be a photo of something that is currently unknown to the general public and I want to thank Magic_lamp for sharing it with us. Please feel free to share what you think this object is in the comments and for those of you that are interested, the largest size copy of the photo is available here for your perusal.