I am trying to install a light switch with a built in timer to replace my original light switch. There are 2 switches side by side in the box but I only want to replace one. The setup currently has 3 main power wires, all with one black one white and one copper wires in each. All the white wires are wire nutted together, none of which are going to the current switch. One of the main power wires has a black wire going to the switch. The original switch had another black wire going back to the box where it was wire nutted to all the rest of the black wires. The new switch that I want to install has one blue, black, and Green wires. I know the green goes to the copper but not sure which wires to connect to the Blue and black wires on the new switch
I'm surprised that the box or paperwork that came with the switch didn't have instructions, so here is my advice (25 years as an electrician): The black wire goes to the hot bundle, the green wire is ground, and the blue wire goes to the light that is being switched. If there is a pig-tail that acted as an individual wire to the old switch, you can either keep that and attach the black wire to it, or remove it from the bundle and connect the black wire on the switch directly to the bundle.
As a way of checking what I am telling you, on the switch itself, there may be some labels for each of the wires (the black, blue and green wires). Typical labeling for the 'hot' wire is: "Hot", "Common", and "Line" Typical labeling for the switched wire is: "Load", "Line", or "Switched"
Note that in my experience, whenever the label for the Hot wire is "Line", the switched wire is never called "Line" -it is almost always labeled "Load" (which is more accurate -but let's not argue semantics).
Good luck and happy wiring!