How to time the blanching of potatoes for freezing
If the temperature drops, you are doing it wrong. The whole point of blanching is to bring up the temperature of the vegetables up very quickly. This means 


slice your vegetables small 
use a very large pot 
blanch in small portions 


A 1:10 ratio is typical, so if you use a 5 liter pot, you can blanch in 500 g portions. Put your vegetables into a strainer. Submerge the strainer in the water (which should be at a roiling boil), start the clock immediately (the water should only need several seconds to get back to full boil) and when ready, plunge into iced water to stop cooking immediately.