No, they can easily get septic and die if you allow them to go too long. Their uterus may rupture if the strain is too great and they can easily die from hemorrhaging or sepsis.
Any animal can reabsorb a fetus if conditions aren't correct, and rats are no different. Fetal size/age determines whether or not it can be reabsorbed.
Do not let her go very long. If after about 12 hours, she's still uncomfortable, lethargic, dehydrated, etc, then you need to start her on antibiotics and let the babies either be passed or take her to the vet(or just euthanize her).
This is why I don't like breeding American Blues, even if they come from a good line. There are always risks with them because of the inbreeding. They are just very poor candidates for breeding...