The picture is very nice. I agree about the blown out highlights. What do you mean by "dark frame"? I would have exposed on picture on the highlights and then masked off the final picture and dropped in the properly exposed one.
With long exposures thermal noise can be seen in images as hot pixels. One way to remove this thermal noise is to take a dark frame and subtract it from the image). This basically entails taking a picture at the same temperature and for the same length of time as the original shot only with the lens cap on (a picture in the dark) and subtract it from the original. For Astrophotography I actual take multiple dark frames and average them together to remove the thermal noise.