Lume the Mad has done the math (very thoroughly), and he's got the answer for Druids: the end healing burst of Lifebloom doesn't cause any threat at all, either for the caster or the recipient. He first pared the entire situation of casting Lifebloom down to its basic elements -- you've got a player who body pulls a mob, and a Druid healing them. He took out all possible reactive abilities that might cause threat, and then set up a situation where the Druid obtained threat, and cast LIfebloom on another player, with an opposing faction Shaman purging the spell early (so it could jump straight to the end heal), and the mob stayed on the Druid -- the big heal didn't cause threat for the recipient at all. Finally, Lum tested if the Druid was recieving aggro, and as you can see above, neither the Druid caster or the Warlock is affected by the end heal of Lifebloom -- just one point of damage can still pull the mob around.
There's been a lot of discussion about this already -- the HoT aspect of Lifebloom still does cause threat (for the Druid), and so you combine that with the fact that lots of people were testing under "unsecure" situations, and the whole thing got very confusing. But Lum's tests seem very clear: Druids can cast away knowing that they won't pull aggro with that burst of healing at the end of the spell.