Why a cold snap threatens a fresh pour
Curing is a chemical reaction between cement and water, not a drying process, and cold weather slows that reaction down hard. Slow it down enough, or let the mix actually freeze before it has gained real strength, and the damage does not undo itself once warmer weather returns. Water sitting in a slab that has not yet cured enough to resist it can freeze, expand, and do real harm to that surface, especially near the corners and edges where a slab loses heat fastest. At this elevation, a hard freeze in the days right after a pour is not a rare event some winters, it is a normal thing to plan around.
What the blanket is doing while it sits there
A curing blanket is an insulated cover laid directly over fresh concrete once finishing is done. Curing itself gives off a small amount of heat as the reaction runs, and the blanket's job is to hold that heat in against the slab rather than let it bleed off into cold night air, keeping the concrete warm enough to keep curing instead of stalling out or freezing. It is one of several things a crew reaches for on a cold weather pour, alongside adjusting the mix itself for the temperature and simply timing the pour around what the forecast is doing that week.
How long it stays down
There is no single number here. How long a blanket stays on depends on how cold it actually gets and what the mix is doing underneath it, and a crew watching a real cold snap will leave it down longer than a mild one calls for. The blanket comes off once the slab has gained enough strength to handle the cold on its own, and normal curing continues in the open air after that, the same as any other pour past its first vulnerable days. Pulling it too early on a genuinely cold week undoes the point of putting it down in the first place. A blanket pulled back to check on the slab should go right back over it rather than sitting off to the side overnight, since a few hours of exposed cold air can undo a full day of protection underneath.