Actually if you have her on your party and use the item, it will work on her too.
And while you can't bring her to the bandits castle, the skeletons on the 3rd floor of the dungeon will do the trick for you.
But for that, you will have to grind her to a decent level, so she dosnt die to them.
Also, there's the fact that they will target Atya too, so it's 50%-50%.
And Finally, she takes a while longer to increase lust, aaaaaaand she faints. So buy lots of HP potions and unlock the teleport portal on the 2nd floor.
A good way to do this is leaving atya at 1 health when you go to the skeletons and leaving the blue haired at full.
The skeletons will kill atya and will be a 100% chance of you increasing the blue haired lust.
This is also effective because if the blue haired girl faints during the battlefuck, and atyua is alive, she will die at 0 health, so you will need to ress her. If Atya is already dead and the blue haired faints, she will exit the dungeon with 1 health, so you just need to heal her and do another run.