Black is where Barragán’s volcanic ground turns from gray to genuinely dark — solidified lava, deep shadow, and the bottom of a pool, not a flat graphic black. Dry lava reveals a granular charcoal texture; wet surfaces turn darker and more reflective; in garden settings it takes on a mossy green undertone.
A dark pool can repeat a colored wall in reflection while its floor stays visually impenetrable — black supplying the depth that keeps a reflection from feeling like a simple mirror, and anchoring saturated colors so they read as architecture rather than decoration.