what are you needing this information for? I can provide information based on families who have encountered this in the US.

Unless something has changed I don't think the United States Supreme Court has made any rulings on mature minors regarding the blood issue.

But for children in general in 1979: “The law’s concept of the family rests on a presumption that parents possess what a child lacks in maturity, experience, and capacity for judgment required for making life’s difficult decisions. . . . Simply because the decision of a parent [on a medical matter] involves risks does not automatically transfer the power to make that decision from the parents to some agency or officer of the state.”—Parham v. J.R.

That is the only one that I know of.

I'm not 100% sure that the above ruling was specifically about the blood issue, but it can be used in arguments over parental rights.

court cases on blood:
Canada: g95 1/22 11-15; g94 5/22 3-8, 12-14; g87 10/22 17-20
United States: g94 5/22 14-15; yb91 59
experiences:
acute lymphoblastic leukemia (baby): w01 4/15 14-16
acute promyelocytic leukemia: g01 4/22 30; g00 8/22 20-21
girl’s zeal in field ministry: w01 6/1 10; w94 7/1 30
young victim visits Bethel: g93 6/22 15-17
youths faithful: g94 5/22 14-15
youths faithful unto death: w08 10/1 30-31; w96 2/1 15; g95 1/22 11-15; w94 7/1 29-31; g94 5/22 3-10, 12-14

Parham v JR wasn't over the blood issue.

Some children over a certain age have been granted the right to speak directly to the Judge in their own defense, but for the majority of minors - it's practically automatic that when the medical deems it necessary to do a transfusion, they will call in CPS if the parents object to the procedure and call up a judge to get approval. The severity and time in which the child has also makes a difference. If it's a long draw disease or problem, and the child has the time and opportunity to talk with a judge but for quickly degrading health due to traumatic injury or rapidly active disease - they more then likely will not give the child a chance.