What does Bridie notice about Ita soon before she begins to convulse? How is this significant?

Bridie notices that Ita's nail beds are turning bluish. Julia says that her face is still red and is concerned about her blood pressure and respirations. As Julia had mentioned the four color stages associated with the flu, with blue being second to last, it seems the comment about her bluish nails, or cyanosis, was a harbinger of Ita's convulsions and untimely death.