Blood is dangerous medicine but you are correct, the main reason is because Jehovah forbids the injesting of blood either way! Acts 15:28,29 28 For the holy spirit and we ourselves have favored adding no further burden to YOU, except these necessary things, 29 to keep abstaining from things sacrificed to idols and from blood and from things strangled and from fornication. If YOU carefully keep yourselves from these things, YOU will prosper. Good health to YOU!”<quoted text>
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.
I want to call your attention to the fact that the inspired Scriptures put the misuse of blood in bad company, things sacrificed to idols and fornication! You may as well worship idols or be a prostitute as to take blood! Not a happy thought is it?