What is the black mould on walls and ceilings?

Several things are black. Black Dust Around Vents, Aspergillus, Stachybotrys (Toxic Black Mold), Alternaria, Cladosporium. Colour is the least reliable way to identify mould, so texture and location are better starting points.

Mould on walls and ceilings

Growth on a wall or ceiling with no obvious water source usually means the water is behind it. That is the case where opening up the wall is justified.

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 on walls and ceilings Green mould on walls and ceilings White mould on walls and ceilings Grey mould on walls and ceilings Yellow mould on walls and ceilings Orange mould on walls and ceilings Brown mould on walls and ceilings Blue mould on walls and ceilings

What it is likely to be

Things that are not mould are listed first.

Black Dust Around VentsNot mould

Follows airflow or the framing pattern behind the surface, in straight lines.

EfflorescenceNot mould

Dissolves in water. Mould does not — this is the definitive test.

AspergillusInvestigate

Powdery, granular surface that puffs when disturbed.

Dry RotInvestigate

Wood cracks into cube-shaped blocks across and along the grain.

Stachybotrys (Toxic Black Mold)Investigate

Wet, slimy appearance rather than the dusty or fuzzy look of common moulds.

AlternariaRoutine

Velvety, slightly woolly texture with a raised profile.

CladosporiumRoutine

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

PenicilliumRoutine

Blue-green colour with a white growing edge.

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 wall or ceiling. 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.

8 possibilities

from 1 of 4 answered

2 of these are 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.