Maybe make the spawned unit die right after spawn. And then detect the unit dies event. Won't be efficient though.
I don't remember there's a way to detect spawned units from Black Arrow.
(For Black Arrow)
Event - A unit spawns a summoned Unit
Condition - Summoned unit has buff Dark Minion
(For Parasite)
Event - A unit spawns a summoned Unit
Condition - Summoned unit has buff Parasite (Minion)
You could give the summoned unit a dummy ability with no icon and detect whether the level of the ability for the summoned unit is greater than 0
You can always add the unit that has the black arrow buff to a group and then every second check to see if it's alive or not. If it's not then create your summoned unit and refer to it via (Last Created Unit).
Also check to see if the buff has worn off and if it has then remove it from the unit group.