L.M.
So sorry for your loss.
In my experience as both an employee and HR representative, insurance companies typically stipulate that additions can be done within 30 days of a qualifying event or at open enrollment. You should have 30 days to enroll your child but the bills from within the 30 days should be covered as well. Since your bills are from within those 30 days, you should be covered.
If I understand what you are saying, they are declining to cover your daughters' portion of the hospital bill for their birth and 10 day NICU stay. Is this correct? I suggest that you contact your HR department head and explain this to them and ask again for thier help. I would also call and ask BCBS if they cover the first 30 days after a child's birth before they are "added" and then go with that. Explain that your bills are from days 1-10 and your girls no longer needed to be added. Therefore, you did not have them "added" after their death since further coverage was no longer needed.
If that doesn't work, contact the insurance commissioner and the press.
Your HR Department should have called you when you missed work due to maternity leave/early delivery. I am sure your supervisor knew why you were out and they should have immediately notified your HR Department who in turn should have send you information regarding insurance (and told you if you still needed to add your girls even though you lost them).
Best of luck to you.