What is the black mould in the bathroom?

Several things are black. Alternaria, Cladosporium, Surface Mildew. Colour is the least reliable way to identify mould, so texture and location are better starting points.

Mould in the bathroom

Bathroom growth is almost always a ventilation problem rather than a leak. If it wipes off the grout and comes back in a fortnight, the extractor fan is the thing to fix.

Start from the colour

Colour is unreliable on its own, but it is how people search — and each of these narrows to a short list.

Black mould in the bathroom Green mould in the bathroom White mould in the bathroom Grey mould in the bathroom Orange mould in the bathroom Pink mould in the bathroom Brown mould in the bathroom

What it is likely to be

Things that are not mould are listed first.

Pink Shower SlimeNot mould

Slimy and smearing rather than fuzzy or powdery.

AlternariaRoutine

Velvety, slightly woolly texture with a raised profile.

CladosporiumRoutine

Velvety, suede-like texture that is dry rather than slimy.

Surface MildewRoutine

Flat and superficial. Wipes off, leaving clean material underneath.

The species matters less than you think

Public health guidance is to remove all indoor mould regardless of type. The CDC and EPA do not recommend routine species testing for ordinary households, because the response does not change: find the water, stop it, remove the growth.
“Toxic black mould” is a marketing phrase, not a category. Stachybotrys is real and does need soaked material to grow, so finding it tells you something useful about your building. But the health effects that are well established — allergy, asthma aggravation, irritation — are not specific to it.
Mould without water does not persist. Cleaning growth while leaving the moisture source is why it comes back. The leak, the condensation or the ventilation is the actual problem.

Call a professional for areas larger than about 10 square feet, for anything in HVAC ductwork, after sewage or flood water, or if anyone in the household is immunocompromised or has a lung condition.

Narrow it down

Already filtered to bathroom or shower. Add the texture and surface.

1.Where is it?

Location narrows things faster than appearance, because each part of a house has its own moisture problem and its own usual suspects.

2.What does it feel and look like?

optional

Texture is far more diagnostic than colour. Slimy, powdery and crusty point in three completely different directions — and crusty usually means it is not mould at all.

3.What is it growing on?

optional

Material matters. Nothing grows on bare masonry that also grows on bread, and a white crust on concrete is almost certainly a mineral deposit.

4.What colour is it?

optional

Last, and deliberately so. Dozens of unrelated moulds are black, and colour changes as a colony matures — it is the least reliable thing about mould and the thing people lead with.

4 possibilities

from 1 of 4 answered

One of these is not mould at all

Things that are not mould are listed first, because that is the most useful answer to get and the one nobody offers.