The pasilla in its truest form is a dried version of the chilaca pepper, although the name is often given to the poblano when sold in the US. The poblano is different to the pasilla, the dried version of which is called the ancho. The pasilla is particularly common in sauces but can be bought whole or as a powder throughout the US, the UK and in Mexico.
Chile negro or chilaca
The pasilla chile is the dried form of the chilaca and can be bought in a milder form, although hotter varieties are available. The chilaca is quite long and thin and at its most ripe is a dark brown colour. The pasilla is versatile in the sense that it can be used in many dishes with different meats and vegetables, especially as an accompaniment to duck.