Pasilla peppers are also called chilaca chile or when dried, negro chile.
It has long, narrow pods of a unique dark green colour which dry raisin black and wrinkled, and hence the name which is the diminutive form of pasa (raisin).
Fresh pods reach the market only in small numbers, and are called chilacas.
The true pasilla is dark green, long, tapering, and narrow, with a blunt end.
In Baja California, where much produce is grown, the poblano is also labelled as pasilla.
Consequently, California markets provide two peppers with the same name: the true pasilla and the poblano, which is heavier and more broad shouldered than the pasilla.
Obviously, they can be substituted for each other as both are mild to medium hot.