Being a knight in shining armour is fun and all, but keeping it shiny can be a headache, plus, that black knight guy keeps showing up and being more bad ass, the solution? become the black knight obviously.
Really though, blackening is a period, durable, rust resistant surface finish, plus it's a lot easier and faster than polishing.
There are a whole lot of different ways to make your armour black, This is the one that I use.

Step 1: What You'll Need

First off you need a good source of heat, I use a weed burner/tiger torch, but in the past I've also used a BBQ and an oven... I recommend against the oven however, because although it worked really well, it also smoked out the entire top floor of my old apartment building.
You'll also need oil in a spray bottle. These days I use linseed oil, but in the past I've used WD40 and Olive oil, both work well.
Lastly, a rag or some sort, and maybe some gloves.
I shouldnt have to say it, but a fireproof surface you don't worry about making a mess on is a good idea. I have a sheet of aluminum on a couple bricks here.

Thats it.

Step 2: Blackening.

So, have you ever burnt something into a pan really good and had to scrub it off? you know how tough that stuff is? well, that's what were going to coat the armour with.

Start off by firing up your torch and heating the surface of your armour. Pretty quick you'll see a line where the moisture in the pores of the metal is evaporating. play the torch across the entire surface until you have removed all that moisture. It should only take a minute or two. I have no science to back this up, but I suspect that by removing that moisture it allows the oil to soak into the pores, giving the finished blackening that much more of a hold on the steel. Also it preheats the metal so the oil will spread into a film a bit easier.

Got that done? OK, put the torch down to the side, grab your spray bottle of oil and unload a bunch of oil onto the metal. Once you've done that grab the rag and wipe it into a smooth coat.

We're halfway done now.

Next grab the torch again, move around to the back side of the armour and start heating. The biggest reason I do this from the back is that it makes it easier to see how dark the oil is getting. Pretty quick it'll start to go golden brown, then a deeper plum brown, next it gets a bit of a reddish hue to it and from there it will darken to a glossy black with some reddish brown undertones.

If you over heat it the oil can start to flake off, if that happens I finish the rest of the piece then come back and spot blacken that spot again.

I've always gone to black, but since linseed oil is a drying oil I suspect that you could stop at any point in the process and let it dry in that colour, I could also be wrong though, anyone here know? With olive oil and WD40 I've found that you need to go all the way to black or it just stays a tacky mess.

I usually do 2 or 3 coats of oil. I lacked a third hand to take pictures of this, but for the second coat I bring the torch back around to the front of the armour and I spray oil onto the armour in the middle of the flame and work my way over the armour till it's completely
covered again. I find that what the flame doesnt burn it forces into a film before it can drip and run. Then I bring the torch back around back and heat it like before.

I let the armour cool after the second coat, then look to see if there is anywhere that needs touching up, if there is I oil and blacken just those spots for the third coat.

Step 3: It's Done

That's It, It's done. Let it cool off till you can touch it and it should be good to go.

Here's a couple of other pieces I've blackened. The spaulders are with olive oil and the trench plate was done with WD40