It is not clear to me that heavy metals are the toxins being removed but they certainly could be. At earthclinic Ted answers questions about oil pulling and heavy metal detoxification. This is the link to their question and answer section: http://www.earthclinic.com/Remedies/oil_pulling_questions.html
With oil pulling removing toxins and opportunistic organisms from the body it follows that the blood is also being cleaned up.
Here at CureZone some people have reported oil pulling with coconut oil because of its antifungal properties.