You can ask the technician if you're not sure, but if the dark stuff is anything significant, chances are laser is still better than electro. If it seems like they're very stubborn hairs and they won't go away no matter how many times they're hit, there might be a laser power/mode adjustment to be made.
When I switched laser places (because I was swindled at the first place) I'd had 7 treatments, and quite a lot of stuff was left. And then, at the new place, where they had the same machine, I told them about the stubborn hairs, they played with the settings, and boom, they started to fall.
Ask the technician. And you could also have them check the depth of those hairs. Sometimes, there's hair that grows so super deep in the skin it's impossible to get with laser, because heat dissipates before reaching the root.