The sloe berries are blue when ripe and they grow on tall bushes/low trees (clearly my grasp of botany is limited) in temperate climes like Denmark. They look like blueberries but are super sour and probably not delicious in their raw state (my friend tried one and I surmised this from the look on her face).
To make a sloe drink, you pick the berries, freeze them for a few days, and then put them in to the alcohol (vodka? gin?), let them marinate for a few days. Then drink.
Last year, when on a bike ride through Amager Faelled in Copenhagen, I noticed this guy picking something from the bushes. After racing over to him like an excited puppy, I asked him what he was doing.
The berries matched his eyes, and he was accommodating enough to model the sloe berries for me.