Bluedot evidently has some odd behavior. Alliant put out a warning not to use it with 125 grain bullets in .357 Magnum, nor in 41 Magnum at any load level. No further explanation, though.
In general, any combustible chemicals storing potential energy will require some amount of energy injected into them to initiate breakdown and release of that potential energy. Since the injected energy is heat energy in powder ignition, the colder a powder is, the more energy you have to inject to reach that threshold, and the more heat energy a cold grain adjacent to a burning grain must absorb to start burning itself. This is what normally slows the burn rates down as they get cold. There are some odd exceptions. IRRC, liquid nitroglycerin, when frozen, is more sensitive than when liquid at a nominal storage temperature. This is probably because it normally combusts by detonation and the solid form carries a shock wave more readily than the liquid form. But I don't know for sure about that detail.
From the above, I think it is fair to surmise that using a magnum primer will mitigate low temperature ignition sensitivity loss some. A magnum primer raises the pressure in the case, which will help energy transfer faster. Most magnum primers also throw hotter sparks. I would test the load with both magnum and standard primers at normal temperature. If you find a magnum primer that raises the velocity a bit. Make a note of how much and when temperature gets low enough for the regular load to be slower by that same difference, switch to the magnum primer that caused the increase.
Edit: Note that many chronographs are built from consumer grade electronics, many of which are only rated down to 0°C (32°F). Sometimes the chronograph will not only fail to function below the freezing point, but the screens can go black. You'll have to try yours to see. Battery failure to deliver adequate current for operation in the cold is another issue. Using Lithium batteries can help, as they aren't weakened by cold as much as standard batteries are.