The reason that blankets are dangerous for babies to sleep with is because when babies are under a year old they can roll over onto the blankets and not be able to move their bodies or face away from the blankets.
Then when they can't move their face and bodies to get away from the blankets which can then cause the baby to stop breathing.
Then since they cannot breath they can die as a result of the blanket suffocating them.
Once the baby gets to 1 year old or older they can safely sleep with blankets but before then you can keep them warm when asleep with some footed Pajamas or blanket sleepers that keep them warm like a blanket but they were it and it's the same as footed Pajamas but much warmer than regular footed Pajamas.