Cast iron is supposed to be black - I think that's the normal reaction of iron and oil. When you season it, you should get a thin, shiny bluish-black coating all over the pan. It might appear mottled but that's fine; you just don't want any rust, silver (unseasoned) or crusty spots. You want to protect this coating and build up the seasoning over time.
Here are the seasoning instructions from the bottom of my Wagner's skillet:
"1. Scour thoroughly
2. Coat with cooking oil
3. Heat in 300 F oven - 1 hour
4. Remove excess oil
Ready for use - reseason as necessary"
One thing I learned the hard way - don't drench it in oil when you season the pan. The first time I wound up with a weird plasticky coating that was sticky and really hard to scrape off. Put some oil in the pan, use a paper towel to wipe it all over (top and bottom), and that should be enough oil for seasoning.
When you start cooking with the pan, you'll want to use plenty of oil to keep food from sticking, or only cook greasy meats or deep fry with it. As long as you treat it well, the seasoning will build up and eventually won't need as much oil. When the pan is young, it's best to keep food from getting stuck on the pan so you don't have to scrub hard and wear down the seasoning. Stay away from eggs for awhile unless you use a ton of oil/butter, and stay away from acids.
For your everyday cast iron cleaning, you need a good scrub brush - not wire, just regular bristles. As soon as you're done cooking clean the pan under the tap with HOT water and the scrub brush. Don't use cold water, or any soap or detergent. Cold water can break the pan, soap will ruin the seasoning. It's normal to have some food stuck on at first; just keep scrubbing with hot water and a scrub brush or scouring pad and it will come off. Oil and coarse salt can help scrub food off too without using soap.
When you've got all the food off, put the pan back on the stove on medium heat until all the water's evaporated, then put on a layer of oil with a paper towel. It's normal to see blackened oil on the towel when you're doing this. Let the pan cool; the pan should have a thin, shiny black coating all over. It shouldn't be greasy - wipe off any excess oil.
It's important to always clean the pan right away and not use soap. You'll probably also want to reseason the pan a few times after every use, then reaseason every so often. If you don't use the pan that often they can get kind of rancid smelling - scrubbing with some oil and salt, then rinsing and reseasoning will help with that.
If the pan needs extra cleaning, try oil and salt for stuck-on food, or salt and a little vinegar/lemon juice for rust spots. The scrubbing and heating dry on the stove will clean just as well as soap. It's possible to clean with a little dish soap occasionally, but you should always follow that up with reseasoning.