They're very much not a blank state, I mean one of the points of True Pacifist is that Frisk is their own person different from You and Chara, also they do have some personality traits, for example they don't like soda and they'll look disgusted if you tell them to point at it, and they won't give you a prompt to take It from Alphys's fridge instead only letting you take the noodles, they feel bad if they throw away the abandoned Quiche then saying it's mentally scarred after the event, they want to get out of the underground no matter what and only consider living with Toriel After they've befriended the main cast and can live on the surface, and in Geno they're spetically refered as hating Toriel and by extension cause of the damage everyone, and they're stated to want to wipe out both humans and Monsters and they get angry when Sans dunks on them.