Why it grows there and nowhere else
Mould needs moisture, warmth and something to feed on. Silicone joints give it all three. The joint stays wet longest because water collects in the corner, it is warm from the shower, and cheap non sanitary silicone contains plasticisers that mould happily lives off. Sanitary grade silicone includes a fungicide, but that fungicide leaches out over roughly three to five years in a family bathroom, which is why sealant that was fine for years suddenly starts spotting.
If the mould always returns in the same short section, look for a joint that is holding standing water or a slight fall in the tray or bath that lets a puddle sit.
Cleaning it safely, and what not to use
A thin bleach based mould remover, sprayed on and left for the time on the tin, will lift light surface growth. A better method for stubborn patches is to soak strips of kitchen roll in the cleaner, lay them along the joint, cover with cling film so they stay wet, and leave for a few hours. Contact time does the work, not scrubbing.
Ventilate the room while you do it and never mix a bleach product with anything containing ammonia or acid, including some limescale removers. Do not use a scouring pad or a blade to scrape the joint, because scratching the surface gives the next growth more to grip.
How to tell when cleaning will no longer work
Dry the joint and look closely. If the black is grey shadowing that lightens with cleaner, the silicone can be saved for a while. If the black is deep, evenly speckled through the bead, or the bead has lifted from the tile or gone spongy, it is finished. No product will bleach it back, and by that stage water is usually getting behind the bead anyway.
Lifted sealant is more than cosmetic. Behind it, water is reaching the join between the tray or bath and the wall, which is the exact route that rots wall boards and stains the ceiling below.
Replacing it so it lasts
The job is straightforward but the preparation decides how long it lasts. Cut the old bead out completely with a sharp blade, scrape the residue, then clean the surfaces with methylated spirit so no silicone oil remains. Let it dry fully. Fill the bath with water before sealing, or stand in the tray, so the gap is at its widest, then apply a single continuous bead of sanitary grade silicone and tool it in one pass. Leave the water in until it has cured.
Skipping the meths wipe is the most common reason a new bead peels within months. Silicone does not stick to old silicone residue.
Fix the reason, not just the joint
If sealant blackens within a year of being replaced, the room is staying wet too long. That points at ventilation rather than the silicone. An extractor that does not clear the mirror within about fifteen minutes of a shower is not moving enough air, and mould will keep coming back whatever you seal with.
We re-seal baths and showers as a standalone visit and we will tell you honestly if the ventilation is the real problem, because there is no point selling you the same job twice.