At the highest level, I would start with freshly roasted whole bean coffee (2-3 wks off roast). Most coffee at grocery stores and even at coffee shops is stale. Like 3-6 months stale. Stale coffee usually has a bitter edge and muted flavor which is why 80% of coffee drinkers choose to add milk and sugar to their coffee. To join the 20%, here are my recommendations:
Start Fresh Buy freshly roasted whole bean coffee with a 'roasted on' date. Ideally, the roast date will be within the last 7 days.
Begin With Medium Roasts Start with a medium roast profile. If you're a black coffee tasting novice, a nice medium roast with notes of chocolate, toffee, and caramel is a good place to get your feet wet. Starting out too dark could lead to smokey and earthy flavor notes. Too light and you'll get fruitier notes. Both ends of the spectrum can be polarizing. It's similar to wine tasting. Starting out to light and sweet or heavy and spicy can be a lot for the palate to take right out of the gates. A few excellent Bean Box coffees I would recommend starting with:
Branch Out You should have nice baseline for what you enjoyed from the medium roasts. Want to experiment with something sweeter, try a lighter roast or an Ethiopian coffee like Ethiopia Yirgacheffe Kochere. Thinking you'd like a bolder coffee, try something darker like Fundamental's Stemwinder Blend.
If you're going to forego milk and/or sugar, I have two suggestions.
Taste the Sweetness If you're new to black coffee, and have a sweet tooth, I'd recommend starting either with a medium blend that's explicitly on the sweeter side, with notes of caramel and/or chocolate. For example, Drip Blend. As an alternative, try a fruit-forward single origin that's more tea-like in overall taste, light roasted Ethiopian or Kenyan coffees are great choices.
Surrender to Darkness Many folks drink their coffee black precisely because they love the kick of a strong dark flavor. In this case, pick a solid dark blend, and brew strong (i.e. slightly less water). Two great options would be any blend from either Fundamental Roasters or Middlefork Roasters.