Thank you for your complimentary words on the blog and for your comment. The argument you make that there is a conflict between norms of jus cogens and immunity seems to be based on two points:
(i) that there is a (procedural) rule that stipulates that third States are permitted to provide a forum in their national courts for cases against other States dealing with violations of jus cogens rules and
(ii) this procedural rule is derived from the substantive jus cogens rule (the prohibition of torture, genocide etc) and is therefore itself a rule of jus cogens. My Oxford colleague, Alexander Orakhelashvili has made similar arguments in his book on “Peremptory Norms in International Law” and his other writings.

I don’t agree that either of these arguments is correct. Sangeeta Shah (of the University of Nottingham) and I are just completing an article in which we tackle these issues (and others) in the context of immunity of officials from criminal prosecutions abroad. Although international law does provide for universal jurisdiction in the context of prosecutions of individuals who commit international crimes, there is little practice or acceptance that States are allowed to enforce norms of jus cogens against other States in national courts. Sure, international law allows for multilateral enforcement of jus cogens norms but the question is whether that enforcement is take place in national courts. We know that the fact that a violation of jus cogens may have taken place does not mean that the consent requirement to the jurisdiction of international courts is overriden (Armed Activities – Uganda v. Rwanda). Your argument suggests that national courts are in a better position that international courts in this regard. The ILC when dealing with Draft Code of Crimes Against the Peace and Security of Mankind was willing to accept universal jurisdiction for most international crimes but not for the crime of aggression. This was because it took the view that national courts prosecuting the crime of aggression would have to determine the responsibilty of the State that committed the aggression and was of the view that this was not permissible under international law.

Even if there are procedural rules derived from substantive jus cogens rule, it is doubtful that these procedural rules are necessarily rules of jus cogens in themselves. For example, there is an obligation on States to prevent genocide. But to say this is itself a jus cogens obligation (because it is derived from the prohibition of Genocide) is to say that it overrides all other obligations that States may have and which may hinder such cooperation. I doubt if the ICJ’s decision in the Genocide Convention case is consistent with that reading.
no general requirement that States provide a forum for crimes or human rights violations which occur abroad. Some treaties do provide for obligatory universal jurisdiction (eg the Geneva Conventions) but in other cases, universal jurisdiction is only permissive. If it is permissive, States will not be violating any rules by not exercising such jurisdctionthere is a rule which separate procedural rules

I agree entirely that there is no conflict between the material jus cogens rule and state immunity, but I am not sure whether I agree that there is no conflict at all. One might also argue that jus cogens rules contain separate “procedural” provisions with which state immunity might clash.
I’ve made this argument a few years ago in a comment to the Greek case you mentioned (see http://www.germanlawjournal.com/article.php?id=271, paras. 17 et seq., particularly 20-22), and I still think that it is valid, at least conceptually. Of course, whether the ICJ will (or should) go for that kind of argument is a different question entirely…

Congratulations on the new blog, which looks very interesting so far!