It is the top blue stripe on AA's fuselage. Plus, I can tell a little bit that the fuselage is silver too.
Sometimes if the aircraft is brand new, and the sun is out, the fuselage looks blue.
Also, American flies into LGW, but not nearly as many flights as LHR gets.
Heathrow gets around 15 flights whereas Gatwick gets only 2 or 3, although I think it may only be 1 now, but I could be wrong.
American's DFW and RDU flights are from Gatwick because they aren't allowed to fly from Heathrow to those two airports. In a perfect world for AA, all of their London flights would go through LHR. The Admirals club at LHR is a beauty. The tarmac view frpm the club is also worth mentioning.
Give me a break, I created this username when I was a kid...