Prevention is better than cure here, so buy well fitting boots and "break them in" by walking in them as often as you can before using them in the mountains. So that your feet get used to the boots and harden a little and also the boots will mold to the shape of your feet to some extent.
Blisters, the reason for these is that as you walk your heels naturally move a little in the boots and this movement causes damage which leads to blisters. The best way to avoid blisters is to buy boots that fit properly and wear two pairs of socks, a thin pair, the kind you wear every day, next to the skin. And a thick pair over them when its cold and or a thin pair if its hot to give your feet more room to swell. The idea being that you wear holes in your socks not your feet !. Walking down hill causes your feet to shunt forward against the front of the boots so keep your toe nails trimmed to avoid sore toes.
If you think you maybe getting a blister stop have a look at your feet and sort it out as it will only get worse if you don't. If its not raining, relatively warm and you have the time to spare then expose your feet to the fresh air for 5 - 10 minutes. Drying your socks out at the same time in the sun if there is any or put socks in your trouser pockets to keep them warm.
If your feet are red or already have small blisters then put a plaster over them and replace the socks carefully ensuring that the plasters don't come off and that there are no folds in the socks at the heel which will rub.
If the blisters are to big to get plasters over and get your socks and boots back on then you should have stopped sooner !, the only thing for it is to burst the blister using a sterile pin and allow the fluid to drain out. Then dry the area because plasters won't stick to it otherwise. During the walk you may need to stop and reapply new plasters if it feels like its starting to rub again.