Quick answer: If the caulk is smooth, firm, and only lightly marked, clean the surface gently and dry the joint. If it is cracked, lifting, soft, or black throughout the bead, cleaning will not restore the seal. Stop before cutting or replacing caulk if water is getting behind the wall or the damage is widespread.
Read the caulk before you scrub
Black color does not tell you exactly what is happening. Soap film, dirt, staining, and growth can look similar at a glance. Check the joint when it is dry. A sound bead should stay attached to both surfaces and feel firm. Gaps, bubbles, peeling edges, and a rubbery section that has broken down point to a seal problem rather than a simple cleaning problem.
Look at the corners, the bottom edge of the tub or shower, and places where water sits. If the wallboard, trim, or flooring around the joint is swollen or soft, deal with the moisture source before focusing on the caulk.
For a small surface mark, keep the clean gentle
Ventilate the bathroom and remove loose residue with a soft cloth. Use warm water and a small amount of mild soap on a non-abrasive cloth, then wipe the joint without digging into the bead. Rinse the cloth often so you are not spreading the same grime from one corner to another.
Dry the seam afterward. A wet bathroom joint can look cleaner for a few minutes and then darken again as it dries. Do not combine bleach, acids, ammonia, or other bathroom products. Follow the label of any cleaner you use, and test it on an inconspicuous section if the surface finish is uncertain.
When replacement is the better answer
Caulk that is split, detached, or discolored all the way through usually needs to be removed and replaced, not painted over. Re-caulking also requires the joint to be clean, dry, and compatible with the new product. If you are not sure whether the surface is tile, stone, acrylic, fiberglass, or painted drywall, check the manufacturer’s care guidance first.
Stop and call a professional if you see persistent water damage, a leak that continues after the shower is off, soft wall material, or a large area of suspected mold. Surface wiping is not a substitute for repairing wet building materials.
For a related bathroom task, see what can safely be removed from the surface of a shower drain. For the basic order of cleaning work, our clean-first guide explains why removing soil comes before disinfecting.