While true Christains may not take whole blood, they may take any of a number of blood fractions that the Faithful and Discreet Slave has determined through their communications with those who have been resurrected to heaven (as seen in the vision given to John by JesusChrist.)
And just to illustrate the above point, let's say your doctor places you on a sugar-free diet. He may then consult with a specialist who would allow you to consume fruits as a part of your diet. Fruits do not contain processed sugar, but fructose. Surely your doctor and the specialist also see the need for you to have other important nutrients that fruit contains e.g water, vitamins, and fiber. This allowance would be out of genuine concern for your health.
P.S - Fructose IS sugar. This illustration is really a load of crap, but so is all the information for the talk too.