Iraqi oil was already on the world market for free under the UN-controlled embargo. Well, technically not for free, we had to send over a fraction of the surplus food our government-subsidized farms accidentally grow, which the homeless people of our country would have let gone to waste anyway.
Going over to take control of the physical means of production (which we would have had to destroy and rebuild in any military invasion) would have cost hundreds of times the shipping costs of that food, and then you have to tack on the costs of extracting it ourselves, refining it ourselves, and shipping it back to the States ourselves. If all we wanted was Iraqi oil, there were much cheaper ways to go about getting it.
There are plenty of valid arguments concerning the war in Iraq. "Blood for Oil" is not one of them.