I always recommend an Americano (or a Long Black). It's an Espresso shot with hot water added. It drinks like a coffee but much of the bitterness is removed and the underlying flavor of the coffee is revealed. With a good quality coffee I rarely feel the need to add milk or sugar.
Everyone has their own opinions, but to me an Americano is the worst form of black coffee. Espresso blends are typically on the darker side (more bitter), and often include Robusta beans in their blends to produce more crema. Robusta doesn't have a particularly great flavor on its own, some (including me) liken the taste to burnt rubber.
I think the clearest way to show someone how different "black coffee" can taste is to go to one of the extremes - pour over, hot or iced, with a light-roast bean from Africa, perhaps with a Natural or Anaerobic Natural process. You can get some pretty insane flavors out of a bean just by the processing it goes through after picking the coffee cherries. They can sometimes blur the lines between coffee and tea flavors.
I guess the difficulty there is finding a shop which uses good beans for their espresso drinks. I'm assuming this theoretical person we're talking to won't be convinced to invest in a full espresso setup, so they have less control over the beans used.
A good espresso shot shouldn't taste that bitter. But like other things, it's an acquired taste. I used to drink only tea, then I learnt to love (or got addicted to) coffee. I started with turkish coffee which is about the most "hard core" coffee I guess so anything else feels subtle compared to that...
