Here's an article that was published in the latest Canadian Medical Association Journal. It has an excellent depiction of how the blood changes color due to methemoglobinemia induced by Sodium Nitrite.
Keep in mind, this blood was drawn from someone who consumed SN, it wasn't added directly to the blood as is commonly done prior to CTBing.
I reccomend you view the attachment on a mobile device so you can see the blood comparison in better detail.