[2021] Umbral blade does not apply shadow dagger appropriately with lethal darkness allocated

As Llama8 stated, Umbral Blades has 40% less chance to apply ailments, so all hits from Umbral Blades and its subskills have less chance to apply Shadow Daggers. Increased Shadow Daggers effect is a multiplier on its damage and has no impact on the chance of application.

There is currently a bug with damage numbers where if two instances of damage occur on a target at the exact same time, only one will be shown. This is why you’ll generally not see the damage from Shadow Daggers triggering. We’re working on a fix.

Also, it’s very counter intuitive that the Blade Explosion subskill scales with Intelligence instead of Dexterity.

This is an errant tooltip, we’ll fix that. It has no impact on gameplay though - the attribute scaling used for subskills is always that of the skill which created it (Umbral Blades, in this case).
Thanks for the reports!

Effects that occur “when you use” refer to a discrete action taken, i.e. when you cast Umbral Blades, you have a chance to gain Dusk Shroud from Dawnfall. The blade exploding afterward is not a discrete action performed by you, so it doesn’t re-trigger the “when you use” effect.
Explosive Blades do have a separate hit, so “on hit” effects can occur again.
Nodes that modify Umbral Blades, such as Edge of Obscurity, do apply to its subskills such as Explosive Blades.