Why is my milk blue?

Have you ever noticed breastmilk have a blue color? This is normal and safe to offer! Blue milk can occur when breast are fuller and a longer period of time has past since nursing and expression. Casein is what causes the blue color and this accounts for 30-40% of breastmilk protein composition. The other protein is 60% whey. Colostrum has a 80/20 ratio with whey/casein ratio, normal lactation 70/30 and late lactation to weaning 50/50 ratio.

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4882692/#:~:text=2.2.&text=The%20whey%2Fcasein%20ratio%20in,in%20late%20lactation%20%5B15%5D.