How do I pre-treat and stain my hardwood floors a dark color?
It depends on what type of hardwood you have.  Some species like pine/maple are almost impossible to stain dark without blotching. 

I have stained oak floors to almost black.  We picked up the darkest stain we could get - sorry I don't know the name but this was 8-9 years ago.  We sanded the floor then went over with a fine sanding.  Then we painted the floors with water.  Not sure how to quantify the water but it took about one hour for it to visibly look dry.  After we applied the first coat of stain.  Just rubbed it on with a rag and another guy was behind me wiping off any excess.  We did this three times total - one day between coats - no one was allowed in house but us.  48 hours after the last coat they threw on the finish/poly.  Looked great but the family had to go on vacation and trust us at their house (grandpa was letting us in).

My advice to you is put a few pieces of the wood material together - if you don't have any pick some up at big box or craigslist.  Go through the process - sand, water, stain, stain, stain - and make sure it looks like you want.  You won't get the perfect black finish with all woods.  You might find that you might want a different tint if you can't get that.  Also I would not do the water if you don't have a hard wood - it will cause blotchiness.  Also putting on gobs of stain will not make your floors darker it will just make it look like a kid did yours floors.  You will need to do more coats to get it darker.  My guess is after 3 coats you start to get diminishing returns.  Some people do 2 coats, some do 3.  Maybe on your test you do half 2 coats and half 3 and see if you can tell.

Edit: On a side note to staining.  If you are doing the floors totally DIY.  Do not over sand your floors with the course grit.  If you over sand with a course grit, you will get swirls.  If you leave those swirls then it will be very important that you get almost black.  Because you will end up with a chocolate floor with very black swirls.  The swirls will take to the stain faster and the pattern will show.  Not trying to scare you here - just go easy on the sanding and do finish sand it.