Final answer:
Blue schist is formed under high-pressure and low-temperature conditions at convergent plate boundaries, specifically where oceanic crust is subducted beneath continental crust. It's characterized by the presence of glaucophane, which gives it a blue color.
Step-by-step explanation:
Blue schist is a type of metamorphic rock that is typically formed at convergent plate boundaries where oceanic crust is subducted beneath continental crust. It is associated with high-pressure, low-temperature conditions often found in these subduction zones. During subduction, the rocks are subjected to large amounts of pressure but relatively low temperatures, which leads to the formation of blue schist and the characteristic blue-colored amphibole called glaucophane.
In a broader geological context, blue schist can also form in regions that have experienced ancient subduction and have since been uplifted and exposed on Earth's surface. Examples of such regions can include parts of California's Coast Ranges or the Greek Islands. However, blue schist, by definition, suggests a specific metamorphic facies indicating specific pressure-temperature conditions rather than a particular geographic location.