As is, the only sustained effects in dark magic are control effects. With how freely CC immunity is tossed around, this means that basically anything not trash can't really have the passive applied against it at all, which overlaps heavily with "things I actually care about crits on".
Some form of debuff or DoT would go a long way here, or changing the passive to "things you've hit with dark magic in the last X seconds", which would allow Crystal Shard and Daedric Mines to apply it (even when the enemy is immune to their associated control effects)