The large white object flying at or faster than the airliners is odd in this respect. White paint on the XB-70 (and British Vulcan) was anti-radiation paint for after a nuclear bombing. The black paint on the SR-71 was to dissipate heat and I suppose not reflect radar as much.
Very few UFO are white, they are typically silver or black from what I recall.
The B1 bomber gave up white paint decades ago in favor of a neutral gray.
Strangest thing in the story is not the airline pilots talking of a disappearance or a visual stealth mode, it's the dang reported color - white.
EDIT:
Note that the B-21 Raider on Northrup- Grumman's website is "BLACK".
http://www.northropgrumman.com/MediaResources/MediaKits/B21/about.aspx
B-21 has a "high-flying requirement"
http://www.thedrive.com/the-war-zone/14919/the-b-21s-three-decade-old-shape-hints-at-new-high-altitude-capabilities
And some DARPA projects are painted white in the testing phase
http://www.aerofatos.com.br/2013/12/where-does-rq-180-fit-in-stealthy-uas.html
What was spotted could have just been the launch plane for a clandestine test?
Boeing Phantom Ray and NASA 747 Shuttle Carrier Aircraft
http://www.boeingimages.com/archive/Boeing-Phantom-Ray-and-NASA-747-Shuttle-Carrier-Aircraft-2F3XC5KZB08.html