Black triangle flying low over northwestern Florida.I walked out back to smoke a cigarette, watched a plane fly over and disappear over the Gulf of Mexico (due south) was looking at Orion, trying to pick out some of the fainter stars. By now my night vision was fairly good. I caught movement out of the left corner of my eye, and startled, I instinctively ducked. At first, I thought it was a hang-glider, basing this initial impression on the shape alone. I was startled, as this had "sneaked" up on me without any noise, no cutting of the wind, nothing. Coming from due east headed due west, this triangle was of a conflicting description. It was darker than the night sky, no lights, but also seemed somewhat transparent in that the outside/frame/shape was more clearly defined than the middle. Black but with the appearance of "transparency" toward the center mass. Contradicting terms, but that's the best words I can use accurately. It was so quiet and stealthy, had I not been looking south at that part of the sky at that moment, I would have never detected it. That part startled me the most. It glided overhead, less than 400' AGL, and remained in my vision for seven to nine seconds until it dropped behind the trees. In other words, moving steadily, but not very fast. I would estimate the width greater than 25 feet. Hurlburt Field is due West of here. If it maintained it's heading for a few more miles, it had to cross it the base and certainly the runway. During my observation, there was no change in altitude or direction, nor change in velocity.