The reason mainstream media doesn't mention that it's against international law is because it's not against international law. So they just aren't lying to you, while alternative media is lying to you.
International law isn't written by the UN or whatever organization you might imagine would be passing world legislation. It's just treaties nations agree to. Nations don't want to make it illegal to do things nations want to do. This is why no one has made war illegal under international law. Countries want to still go to war, so it's not illegal under international law. Countries want to be able to do blockades, so blockades are not illegal under international law.
Sure you can say you don't like it, you could say a blockade is wrong, immoral, heinous, or whatever emotional terminology you'd like. But saying a blockade is illegal is either ignorant of international law, or just plain lying.