http://i.imgur.com/y3FHGNw.png Best I could do for a direct source we can all trust 99.8% of the time (been discussed before and someone tested it. Pretty sure it was MMO-C hunter forums you can dig into), point is that that all DoTs/debuffs/stuff the player applies to the target as a debuff snapshots on hit. It's like this with all dots in the game iirc - the fact that it applies for Serpent Sting should be a decent enough source, though. (That's from the EJ BM guide so it didn't list Black Arrow, checked SV one but it's not nearly as well done).
If you've been around for any decent amount of time you'd kind of have read threads about it, though. Not trying to be a jerk either but naked character or not, Blood Fury is a pretty weak cooldown to use it with.
As for knowing it, it's not really a big deal.. Like at all, I can't think of many cases where this knowledge is really relevant and the few cases I can come up with are RPPM-based randomness.