Nevermind, I agree with Gloombunny. It says you need to be using a weapon from the dagger, crossbow, or sling weapon group. It does not say "implement". If it were to say simply using a dagger, crossbow, or sling, then that'd be cool. But no, it's saying a weapon, therefore the dagger needs to be used like a weapon. Which it is clearly not, as the power does not include the "Weapon" descriptor.
Edit: You do not need to ranged basic attack with Eldritch Blast for it to count as an attack that could potentially use Sneak Attack. I have no idea where you got that idea from. However, Eldritch Blast is invalid for the reasons Gloombunny and I stated.
Edit: You do not need to ranged basic attack with Eldritch Blast for it to count as an attack that could potentially use Sneak Attack. I have no idea where you got that idea from. However, Eldritch Blast is invalid for the reasons Gloombunny and I stated.