Our early legal system was based in English common law, which Blackstone had brilliantly described, so it was not unusual or misguided to refer to his writings in early legal interpretations. This is quite different from now just jumping across the pond or around the world to find things to support the way you want to rule when you can't find another way to get there.
Thanks for your help with some facts on Blackstone.
I suggest you read the decision in Roper v Simmons. The court referenced foreign sources only in the very end and the court noted "The opinion of the world community, while not controlling our outcome, does provide respected and significant confirmation for our own conclusions." Far from not being able to "find another way to get there", the Court, in fact, based their opinion on U.S. sources and precedent.
And one thing that people continuously ignore is the fact that in Roper v Simmons the court was upholding the decision of the Missouri Supreme Court which found that executing somone under 18 was unconstitutional.