In this context the UN is simply the governing body of international law. It holds no mechanism of enforcement and is wowed to non-interference and impartiality. When the UN finds a blockade illegal it simply means that it violates the international laws it self has set. You can think of this like a supreme court ruling inside your own jurisdiction, just between states as opposed to people.
There is no bias here, just law and interpretations on these laws.
If this were the security council that would be another issue however.
