The nurse comes in with several pint glasses of blood and tells you to drink it all. Yuck you would never do that. Now explain the difference!
Blood isn't intended to be ingested in this manner. The best way to explain the difference is by analogy:
Let's say you had a terrible case of hemorrhoids and the nurse told you that, instead of inserting a suppository into your rectum, she told you to suck on it. Yuck. You would never do that.
It's faulty to suggest that Jehova's Witnesses decline blood transfusions because it's comparable to drinking blood. That's insulting to practicing Jehova's Witnesses.
It's also faulty to suggest that the reason behind declining blood transfusions is related to the threat of HIV, STDs, etc... It goes much deeper than that.