If you don’t want to change JUST deteriorate and dark pact, which i think is a pretty significant change that will have some fairly significant effects to the blue matchup, I recommend changing Arresting Constable and Guardian of the Gates.
Arresting Constable is simply changed to a 4/5 or a 3/4. If that isnt enough, then lowering its cost to 3, but i doubt that would be needed.
For GotG, simply changing to 2/6 instead of 1/6 helps a LOT specifically in the black matchup. GotG should be comparable to Voidblocker, but in my opinion, GotG should be more powerful. As it is, voidblocker has an effect on heroes, and effects how someone chooses to order their attacks, and can go off even against swift strike or obliterate. GotG is meant to force a hard stall to the opponent. In its current form, it rarely slows black down at all. Just adding heroes to the effect would probably be sufficient if the increased attack isn’t enough.