Molds thrive in very humid, damp environments. As a result, when colonies of black mold thrive, they will tend to produce a ‘moldy’ or ‘musty’ odor.
What you are actually smelling is the by-product of the mold decomposing whatever organic material it is consuming in order to survive. Moldy odors will typically have a rotten smell, such as rotting vegetables or wood. Some people have also referred to the odor as being earthy (rotting wood), like sweaty socks, or damp musty carpet. In all cases the key ingredient is moisture. If you notice a moldy, musty smell, chances are you have a moisture problem (leak) that has created the perfect breeding ground for mold (first place to check are hidden areas in your basement).
Molds are extremely small (microscopic) and begin producing toxins (a by-product of their digestion of organic materials) as soon as they establish themselves in a damp environment. What this means to you, is that you won’t be able to detect any unusual odors until the colony grows large enough to produce enough toxin for you to detect it. When that happens, your best course is to track down the source of the moisture and eliminate it.
It may sound obvious, but molds smell ‘moldy’, like something is decaying in a damp environment, which is exactly what is happening.