It's supposed to add the Black Arrow buff onto the targeted unit (or attacked unit if Black Arrow is on Autocast), but it doesn't do so, even if Targets Allowed has Hero included in it. You could place a dummy buff and add the unit to a unit group, then remove it from the unit group after a duration (I measure these with dummy units attached in hashtables). Then check every unit that dies to see if it is in the unit group.
Have you tried not using the Black Arrow base buff? It may just be the nature of the base buff itself, so a different buff will work. Either that or it's the Black Arrow ability itself that is causing the problem.
I do know this - xenforo dropped the ball by not keeping the vbulletin reputation comments as a feature. The loss of the Reputation comments data when we switched to Xenforo really was the death knell for the site when it came to all the users that left. I know I missed it so much and I got way less interested in the site when that feature was gone and I run the site.
I'm on a page about incorrect corrections, and spent the better part of like two hours trying to get someone to understand that -5^2 = -25, not 25, and then that post had comments get reposted because that group is self sustaining, and that person was in turn trying to explain what I just explained to them. And I'm taking that as a victory