I first thought that using a Warlock power with a pact blade would work for sneak attack because you are essentially using a light blade implement. However after reading sneak attack again I see that it requires a weapon that is a light blade.
So RAW it does not work... however as a DM I could be persuaded house rule allow it...
Exactly.
I would consider allowing it as a DM, but I'm not sure. It's definitely not RAW, but at the same time it seems like it's not really a necessary limitation given that you can use the multiclass hunter's quarry. I suggested earlier reflavoring the quarry, but if a DM wants to allow a sneak attack to be used with the blast, that's his call, and wouldn't be totally out of line.
If a DM is on the fence whether or not to allow this benefit (as I am), they could consider requiring a feat or some other cost.