Why does the silicone sealant around my bath go black?

That black staining is mould growing in and on the sealant, not dirt, which is why scrubbing barely touches it.

Silicone itself is not food, but the soap scum, skin oils, shampoo residue and body fats that build up on it are. Add permanent moisture and warmth and you have close to ideal conditions. Once the mould has rooted into the silicone rather than sitting on top, no cleaner reaches it — the growth is inside the material, and bleach only removes the surface colour temporarily.

What actually helps:

Ventilation is the real fix. Run the extractor fan during and for a good fifteen to twenty minutes after every shower, open a window, and the sealant will stay clean for years rather than months.

Wipe the seal down after showering — a squeegee or cloth along the joint removes the residue the mould feeds on.

A mould remover left in contact, rather than sprayed and wiped, works on early growth. Soaking a strip of kitchen roll in the product and laying it along the joint for a few hours gives far better results than spraying.

Once it is rooted, replace it. Cut the old bead out completely with a sharp blade and a sealant remover, clean and dry the surface thoroughly, and apply new sealant. Use a sanitary-grade silicone containing a fungicide, and note that the fungicide is consumed over time — it delays regrowth rather than preventing it forever.

Fill the bath with water before sealing and leave it full until the silicone cures. An empty bath rises when you get out of a full one, and a bead applied to an unloaded bath tears at exactly that point.

Regrowth within months usually means a ventilation problem, not a sealant problem.