Large black rectangle with no lights, moving slowly, followed by what appeared to be a helicopter

I saw a slow moving black rectangle in the sky, tonight. We saw it for about 10 seconds, and it was between 8:30 and 8:40 PM. It was in the sky east of PDX airport north of the Columbia River. It was moving to the north and completely black, no lights. However it was followed or accompanied by what I think was a helicopter that did have blinking lights. I judged it to be about the same height as the fuselage of a 737 and a bit shorter than a 737. This is based on my assumption that the following aircraft was at the same distance as the rectangle.
 We were driving west at dusk on SR-14. It was silhouetted. I did see the lights on whatever was following it, but no lights on the rectangle.
 It caught my attention because it was moving too slowly for a fixed-wing aircraft, but was bigger than a helicopter. At first it didn’t register and then I said to my wife “what was that thing?!” She had seen it too but by that time it had gone out of view.
 Shape was a rectangle with sharp corners, and no bumps, wings, rotors or anything, at least not that I noticed.