U-Shaped, strobe like blue/white flash in the sky, with no aircraft seen.My Dad and I were traveling together up a road to the top of a Mesa. We are both USAF veterans. Most would consider us as reliable trained witnesses. The sky was clear but dark as sunrise was not to occur for awhile. Please note the time of this sighting. We were on a mountain trail approximately 9 miles East of Springerville and 5 miles South of Highway 60 and 1-1/2 miles East of the New Mexico Sate border. We were climbing to attain the Mesa top by sunrise and hunting Elk at the time. In the Eastern sky, towards New Mexico from our position, at approximately 35 to 40 degrees above the horizon, a brilliant flash occurred. It covered an area of approximately 1/10th of the sky. The color of this strobe-like light was blue/white and in the shape of an upside-down U. We both stared at the sky, awaiting some other sight, looking for the object that made this strobe-like light, however the sky was clear, full of stars, and no object was seen, only the impression of the light in our eyes, as it does when a flash is seen... it re-appears and and fades for a few minutes afterwards which is normal. We were hoping to see an object that could have made this "blinding" flash, but nothing could be seen. No silhouettes, no shapes, no sounds, nothing. We discussed this sighting and neither of us have any experience with an aircraft or military flying object that could have caused this flash to occur. Whatever it was, it is still unknown to us at the time of this report. Please note: This is not the first report of an unidentified sighting in this area that I have given.