Edit: Due to some recent feedback, I've been thinking of a solution that would be best for both PvE as well as PvP, in regards to the DoT-spread mechanic/interaction between Explosive Shot and Black Arrow(during Rapid Fire). For PvE, this has potential to be very rewarding and fun at the same time.
But, for PvP, you wouldn't want to have automatic AoE built into your ST "rotation".
Now, there are a few possible solutions to this issue.
And the one I've come up with that feels rewarding for players that engage in PvE as well as for those in PvP, is this:
When in instanced PvP, your Explosive Shot will no longer be able to automatically spread any periodic damage effect caused by Black Arrow onto other nearby enemies. Instead, you now have an additional passive effect that is always active, regardless of if your Rapid Fire is or isn't.
That effect is as follows:
If your Black Arrow is at any time dispelled or otherwise removed from an enemy target, you will instantly gain a free charge of Black Arrow.
Note: This is not a CD reset mechanic. This works like Lock and Load does for Explosive Shot, that it would add an extra charge of Black Arrow on top of what you already have.