Ignoring if you can see anything on the screen, when you saw the orange/amber light going on and off slowly, keep holding it until the light should become steady.

If I remember correctly when I last flashed my device, I had to get the device into steady amber light. Mind you however that my screens do work so I could actually see the USB icon on the screen remain static.

Also whilst you have held the device's `U' key what did the computer say? did it still refuse to see the device? what if you say for example created a simple loop to run the same command without any pauses as you try to get the device into flashing mode?