Poblano chiles are a beautiful dark green, almost black chile. Large and luscious, the poblano is one of the most popular chiles used in Mexican cuisine. The typical chile is at least 4 to 5" long. In California, these chiles are called pasilla. This is not a chile to eat raw but use it roasted, rajas (strips) sauces, or stews. When dried, the poblano is called an ancho or mulato chile.
Depending on where you live, they may be a standard grocery store item. Otherwise, look for them in Mexican markets.
If you can't find poblano chiles you can substitute one of these chiles: