I do not recommend the hot oil or "oil blackening" process, and I think the OP has a very poor understanding of what is involved. Oil blackening was used extensively at one time by Springfield Armory for Krag and early M1903 receivers. It produces a nice black color, but was done as part of the heat treatment process and could never be re-done (receivers that had to be reblued were rust blued).
Heat bluing of small parts is common, also usually a by-product of heat treatment (Luger ejectors and safeties for example, which were made a straw color, or Krag extractors, which came out a lovely blue.)
Both types of bluing require heating the part to a level that can draw the temper and leave the part soft or, conversely, brittle; either would be dangerous in something like a rifle receiver.
The assessment of the various "cold blue" treatments is correct; none will give a good and durable blue. The only two methods that will are hot salt bluing, which I don't recommend for home use, and rust bluing, which gives good results but does require a means of heating (boiling) the barreled action.
Today, there is another alternative, which is very DIY friendly - paint. There are a number of coatings on the market (check with Brownells) which are easy to apply, durable, and can be had in various colors to duplicate bluing or Parkerizing if desired.
BTW, you don't put a wood plug in the barrel and then use a screw to take it out, you use plugs that are long enough to stick out both ends so you can use them to handle the action without getting burned or leaving fingerprints.
Jim