Thanks for sharing! I'm glad all the mice are thriving.
What good rattie moms.
I've done the opposite before. Putting a rat fuzzy with a mouse mother. Worked out great!
And a trick in the research lab for integrating babies is have the mother pee on the new baby so it smells like her or put the new baby at the bottom of the baby pile so it smells like the rest of the litter.
Maybe you're seeing the placenta? Moms eat the placenta after birthing each pup. Each pup has their own placenta. It could just look like a lot of blood, but it's not.
I wouldn't worry about it yet.