The reason black mold grows on your shower caulk is that your caulk often retains water and soap scum, which contains cellulose. If left untended for a few weeks, black mold will begin to germinate, especially in a warm dark bathroom.

Why is my shower caulk black?

Shower caulk is usually found around the edges of your shower and bathtub as a sealant. Black stains on shower caulking are often attributed to the growth of mold in your shower. Mold thrives in moist, warm places such as shower stalls and quickly spreads to tile as well as caulk.

Does caulk turn black?

As for the clear caulk that has turned black, that black is mold and is probably under the clear caulk, but it can migrate. Dig all caulking out with a sharp pointed tool, then dig out all old grout, treat that joint with full-strength bleach, rinse, and let dry.