I've got a trigger guard and screws I'd like to get polished and blued for a project rifle I'm working on. Either no one is interested in doing just these parts for me or they want to charge an arm and a leg.
If anyone is set up to do this for a reasonable price please shoot me an email. Thanks!

Were the parts previously blued ?
How old ? Older firearms.. pre 1970 ish, Can have some wildly different heat treats on the receivers and barrels. Along with occasional inconsistent metallurgy. Some older receivers will not take current hot bluing well. Bluing salts are caustic. It is controlled oxidation. I have seen receivers that absolutely can not be blued with common bluing solutions.
If the parts were previously blued then there should be no problem. Any compentant firearms refinishing company should be able to do it. Most shops send them out. If it is an older gun with other blued parts do not expect the refinished parts to match. The shade will most likely be off.

Probably your best bet to have them match your gun's finish is to have them nitre blued, as this process works great for small parts and allows you to monitor the color change. Simply stop dunking the parts in the nitre salts when they're the color you want, or leave them in until they get the classic royal blue color.

Of course most shops do not do bluing or even nitre bluing in house, as the fumes from the salts are incredibly acidic and will ruin any metal exposed to them. Brownells does sell a "small" 10lb bucket of nitre salts that you can use with a Lee lead pot or even on the stove in a pyrex dish (make sure to have a fan pulling air outside). It's not very cheap (about $75 shipped) but is sometimes cheaper than what a shop's send out fee would be, and gives you an almost infinite supply for bluing most small parts.

For nitre bluing you can use salt peter, (potassium nitrate).

It can be found locally sold as stump remover, (check ingredients closely), or on E-Bay

It is a hot, hot procedure, but a common household stove will reach that temperature.

Cant be used on anything heat treated, as it is too hot.  No modern receivers, for example.

Looks nice, like other finishing methods, the preparation is the key.

Nitre bluing is nice, but not durable.

Express bluing can be done at home and is more durable than hot salt blue.  Buy at Brownells.

You have to hand polish the metal (progressively finer grades of emery paper).  Then degrease.  Apply Express blue.  Throw in pot of boiling water.  Card.  Then degrease, blue, boil, card.  Repeat until desired blue is reached.  Oil and allow to cure 24 hours.

What about rust blueing?

You find a way to make the part rust (coat it with some kind of an acid, laundry bleach works), let it rust a bit, boil it in distilled water, card it, and repeat if needed. It works, it's durable, and cheap (bleach is really cheap)

I blued a Remington 700 recoil lug this way. Did the process a few times and got it nice and black.

Check with Hot Flash, they've done a few small part lots for me from time to time.