Grout Turning Black or Dark in the Shower — New York, New York

Shower tile grout that has turned dark gray or black is almost always caused by mold and mildew that have penetrated the porous cementitious surface. Grout is naturally absorbent, and the warm, humid environment of a shower provides ideal conditions for fungal growth — especially in corners, floor joints, and areas where water pools after each use. Surface-level mildew can sometimes be scrubbed away, but once the staining reaches deep into the grout pores, topical cleaning alone cannot restore the original color. Beyond aesthetics, compromised grout allows water to seep behind the tiles and into the wall substrate, which can lead to hidden mold growth on drywall or backer board, structural softening, and eventually tile delamination. A professional deep-cleaning service can steam-clean or chemically treat the grout lines, re-seal them with a penetrating sealer, address any silicone caulk joints that have failed, and recommend ventilation improvements to keep humidity under control.