What adders are you talking about, exactly, that can be added to a warlock’s pact weapon attacks and can’t be added to eldritch blasts?
...
3 more damage per round at the cost of needing at least 13 Strength and being restricted to melee? Seems perfectly balanced to me.
Eldritch Blast:
Agonizing Blast
Hex
Pact of the Blade:
Pact of the Blade
Thirsting Blade
Lifedrinker
Hex/Spirit Shroud
Technically Spirit Shroud is usable with Eldritch Blast, but you basically need to be in melee range to do so, so it doesn't really make sense.
Hex upcasts to longer duration. Spirit Shroud upcasts to more damage. Since this is a question of damage done, Spirit Shroud does substantially more damage starting at level 9.
The pact weapon can do more damage than Eldritch Blast, and also gains the benefit of a weapon mastery. I'll use Graze.
Then add Lifedrinker.
Note: A +1d6 to damage on a weapon is roughly equivalent to a +2 weapon.
Assuming a base 60% hit rate (which will be the same for both, since both sides use Cha for the attack/damage stat), and 18 Cha at level 5, 20 Cha at level 8:
Eldritch Blast:
2 attacks at level 5 (Hex) = (5.5 + 4 + 3.5) * 2 * 60% + (5.5 + 3.5) * 2 * 5% = 16.50 DPR
2 attacks at level 9 (Hex) = (5.5 + 5 + 3.5) * 2 * 60% + (5.5 + 3.5) * 2 * 5% = 17.70 DPR
3 attacks at level 11 (Hex) = (5.5 + 5 + 3.5) * 3 * 60% + (5.5 + 3.5) * 3 * 5% = 24.75 DPR
Pact of the Blade:
Greatsword (Graze)
2 attacks at level 5 (Hex) = (7 + 4 + 3.5) * 2 * 60% + 4 * 40% (Graze) * 2 + (7 + 3.5) * 2 * 5% = 21.65 DPR
2 attacks at level 9 (Shroud, Lifedrinker) = (7 + 5 + 9 + 3.5) * 2 * 60% + 5 * 40% (Graze) * 2 + (7 + 9 + 3.5) * 2 * 5% = 34.95 DPR
3 attacks at level 11 (Shroud, Lifedrinker) = (7 + 5 + 9 + 3.5) * 3 * 60% + 5 * 40% (Graze) * 3 + (7 + 9 + 3.5) * 3 * 5% = 52.23 DPR
So the difference in damage is about 5 points per round at level 5 (not a concern), 17 points at level 9, and 27 points at level 11. At levels 9 and 11, the weapon is doing twice as much damage as Eldritch Blast. If the Pact of the Blade setup just had 2 attacks per round at level 11, it would still be doing more than Eldritch Blast, by about 10 DPR (ie: the level 9 calculation).
And this doesn't factor in Eldritch Smite, though if spread out over the attacks needed to get a crit is just +2.7 DPR.
For reference, I have the Champion fighter currently doing 42.88 DPR at level 11 with a PAM setup.