At this point the difference is OK, the problem is once EB has four blasts.
EB gets its 4th blast at 17th level. Hardly reason enough to dismiss an entire pact in most practical situations.
All the way up to 10th level a typical Warlock can expect EB to do the same damage as a Pact Blade, d8+stat (Rapier for comparison).
Assuming of course the Warlock doesn't take a single level of Fighter for Dueling (or Great Weapon if you're doing something squirrely) style. Either of which would put the damage ball back in the Pact Blade's court.
11th level is a deadzone where EB is patently superior. 3 blasts at d8+stat.
12th the Pact Blade is d8+stat+stat, which can be competitive with EB given the blah blah blah who cares.
All of this discounts magic weapons, which are likely to be plentiful enough to make Pact Blade more than hold its own all the way to 17th and perhaps beyond.
Really, my major beef with Pact Blade is that it doesn't come online until 3rd level. If I build a Pact Blade Warlock, then from the outset I'm imagining some sort of gish. Why the hell do I have to wait until 3rd level for something as basic as what the Pact itself provides? Though the same could be said for any of the Pacts really.
Can we talk briefly about Hunger of Hadar?
How is something that is
explicitly a Warlock spell not capable of scaling to a 5th level slot? That's moronic right?
Edit: This whole thing is a lie. EB is d10.