Asphyxiate is a physical ability.
Unholy Blight should not go through cloak either, if so then it is also a bug. Regardless, it's rare to see a DK take it, because the two other choices in the tier are almost always better for any situation.
Maybe your latency is too high.
ermm, Unholy blight is the first choice in arena? unless you were born under a bridge and had to sleep with black men to feed you 4 year old infant daughter?