Almost every mould job is a water job that was never finished. A basement that flooded and got "dried" with a box fan, a slow leak behind a shower, a chronically damp stone foundation — six months later there's staining on drywall, a musty smell, and someone in the house with worsening allergies.
Proper remediation means containment (so spores don't spread through the house during the work), removal of colonized porous materials, HEPA vacuuming and damp-wiping of everything in the containment, and fixing the moisture source — otherwise it simply grows back.
Colour doesn't determine toxicity — plenty of harmless moulds are black and some problem species aren't. Any significant indoor growth should be removed and its moisture source fixed, regardless of colour.
Bleach whitens staining on non-porous surfaces but doesn't kill roots in drywall or wood, and the water in bleach can feed regrowth. Porous colonized material needs removal, not spraying.
Musty odour with no visible source, staining that returns after painting, or a history of leaks/flooding in that area. A moisture scan plus a small inspection opening answers it definitively.
Not if the moisture source is fixed — that's why source diagnosis is part of every job we do. Mould needs water; remove the water and it cannot return.