Morello cherries are often spoken of as though they are a single cherry variety, but morello is actually one of two sub-groups of sour cherries.
The morello group has fruit that is so dark-skinned it is almost a mahogany red, with dark red flesh and dark juice inside. Sometimes morello cherries are so dark red that they are almost black in colour.
This contrasts with the other group of sour cherries, “amarelle”. Those cherries have paler skin covering flesh that is not red and give off a juice that is almost clear.
Morellos are slighter tarter than amarelle.
When you do see “morello cherries” referred to as though an actual cherry were named that, chances are the writer means English morello cherries or one of several varieties of Hungarian morellos, all of which are highly prized.
Other varieties of morello cherries include Balaton, Fanal, Heimanns Konservenweishsel, Kansas Sweet, Kelleriis 16, Koroser Langenstein, Krassa Severa, Northstar, Oblacinska, Rheinische Schattenmorelle 226 and Schattenmorelle.