I'm under the impression dual blades have lower base dmg for the same level. So that means lower ability dmg.
That appears to be true, but sometimes only by a very narrow margin.
It's kind of hard to compare -- I couldn't find two daggers of the same level and rarity. It's not really fair to compare a dual-blade common dagger to a rare (blue) single-blade of the same level. Also, the lowest level dual-blade is level 12, which means you don't even get them until mid game or at least late early game.
So I sampled a couple of common pairs that are close in level and took their lowest base damage and divided by their level to get an average. If SB is always better than DB level for level, then comparing averages should show SB is always higher than DB, even though they are not exactly the same level.
Tevinter Dagger (SB) Level 11, lowest base damage 58, average is 5.27
Crow Blade (DB) Level 12, lowest base damage 62, average is 5.16
Carta Claw (SB) Level 20, lowest base damage 99, average is 4.95
Whirling Deaths (DB) Level 19, lowest base damage 70, average is 3.68
I guess because DB is AoE they had to give it a bit of a discount in base damage, otherwise it would be too OP level for level.
But none of the above really matters if you have one dagger, of either type, that clearly has a higher base damage over all the rest at hand. You are not going to take an SB Hook Blade at 65 when you could have a DB Knightslayer at 74.