Sure they did. The majority decision refers to the President's constitutional positions as Chief Executive and Commander-in-Chief of the Armed Forces, and do not dispute his power to declare the blockade as a result of this. The court also makes note of the fact that the President cannot declare war on a foreign power and cannot invade a state of the Union. President Lincoln did none of those things. The court makes it clear that the President is authorized to call out the militia in the event of invasion or rebellion. And the court agrees that is what happened. And that Lincoln's actions in combatting the rebellion were Constitutional.
You seem to claim that because the court references 'jus belli' that this somehow confers recognition of the confederacy as an independant entity. Nothing could be further from the truth, and the court makes this clear when it saus, "The parties belligerent in a public war are independent nations. But it is not necessary, to constitute war, that both parties should be acknowledged as independent nations or sovereign States. A war may exist where one of the belligerents claims sovereign rights as against the other." THe court goes on to state that just because one or the other side of a rebellion choose to concede belligerent rights to the other side does not automatically confer recognition, either. The U.S. never recognized the independence of the confederacy. Neither did any other country.
...their dicta carries with it the weight of law...
Actually it does not. It merely represents an idea of the courts belief in such matters. It has no weight of law.
I see in your apologetics, you also failed to cite the constitutional authority.
Actually it does not.
About this you are right. My phrasing was inaccurate.