I'd recommend getting some basic electrician knowledge before fiddling around too much with the 120V supply going into your computer.
What your asking about is a very simple wiring job, and a junction block isn't required to perform it, as there are many ways to do it.
It is one of those things where errors on your part can cause things to break or catch on fire if not assembled in the proper way.
The black cord that runs to your PSU and Monitor is simply a fancy extension cord with a specific plug on the end. The electricity it feeds into your computer is the exact same electricity that is in your wall outlets.
That said on the PSU side of that connecter it plugs into often has components soldered to the leads coming out of it. Not always but quite frequently.
All that is really required is to parallel the matching pin connectors in the sockets with wires. No special modifications or components are needed.
However good electrical practices are required for those connections as you are intending it to be a long term solution.
Talk to me about it on TS3 sometime and I can go over some good options and practices for you, or better yet talk to an electrician friend (as in a professional, unless you are very confident in his DIY skills) and get his advice.