I was in Sapporo and Otaru for christmas 2017.
Expect highs of 8 degrees and lows of -6 degrees. May get colder if theres wind and snow. Roads and pavements are not fully ice yet but may be good idea to get a nice pair of walking boots. from my experience, a wool jacket is a must. My wife wore 5 layers with UQ heat tech compared to my 3 layers with wool jacket. Furthermore you're planning for Asahikawa, prob could get colder.
I did not go to blue pond (still my biggest regret), but i dont think it'll be frozen in mid-dec. Jan and Feb will be frozen as its the height of winter. if you have instagram, you can see posts that are geo-tagged to blue pond during December and check if its frozen or not.
Might want to check out Mt Moiwa. I went there up on a cloud-less night and it was beautiful. Saves you a trip from going Hakodate
If you do go to Hakodate, might want to try Lucky Pierrot. The only fast food available in the region. look out for "Classic Sapporo" beer, i believe its not available in any other parts of Japan as well