In my opinion the cheapest, simplest way to experience a significant jump in quality from what you're currently doing is to buy an Aeropress.
You can get one new for only $30-40, and after trying a few times you'll likely be blown away by how much better the coffee it produces is than what you've grown accustomed to with the Keurig.
An electric kettle is another good purchase to go along with your Aeropress, and it generally comes in handy for plenty of other uses too.
If you start getting into making better coffee at home with the Aeropress, then the next step on your journey would be to invest in a grinder and make the switch to whole beans. That's really the last thing you'll need if you're trying to max out coffee quality at home.
As for drinking it black, that's totally up to you. Personally I like a little cream and sugar in my coffee, and I'm probably seen as a coffee snob by my friends. Don't let anyone tell you that black is the preferred way to drink coffee. It's all preference, and I like the heavier body that cream/milk gives the brew.
If you end up taking any of this advice, then here's my go-to recipe for the best Aeropress coffee:
- Set the Aeropress up inverted (you can Google this)
- Between coarse and fine grind (two scoops)
- Heat water to 195°F
- Pour water in until it just covers the grounds to let it bloom, then wait 30 seconds
- Fill the Aeropress up to the top with more water
- Stir for ten seconds
- Let brew for one more minute
- Press slowly into mug
- Add cream/sugar to taste
I hope that helps. Just be warned... Once you go down this path, you'll likely no longer be able to tolerate K-Cup coffee.