Black mold on carpet indicates a moisture problem, regardless of what it looks like. Black mold on carpet tends to grow in a pattern that will indicate the path of the water, if there is any, under the carpet.
Black mold on carpet can be black, gray, green, yellow or white. The texture may be velvety or leathery. Generally, it will not be "fluffy."
If you suspect that you have black mold on carpet, then its effects can also indicate that you have diagnosed the problem correctly. If you or a family member are experiencing respiratory irritation---particularly in the affected room---this can indicate that you have black mold.
There are two types of black mold that might grow on carpet. Unfortunately, they look very similar. Toxic black mold tends to grow in a thick, speckled pattern stemming from the wettest area. Cosmetic black mold may be more scattered and it tends to have more variety in color.
All types of black mold can cause health problems, though toxic black mold will make a home uninhabitable. You must remedy a situation of black mold immediately in order to avoid chronic respiratory problems.