The only time I've seen this is when working in dusty environments. I worked in a gold mine for a few years and being underground there was a lot of dust in the air at times. This resulted in particles of dirt and dust being found in the mucous.
I'd suggest seeing your Dr about this as there can be some serious consequences with certain things that can be breathed in. Silica, for example, can cause scarring of the lining of the Alveoli.
Something that helps with purging mucous is sea moss.
This is key to what Dr. Sebi spoke about when it came to keeping your body healthy. Mucous is the problem — get rid of it and you'll feel better.
Sebi spoke about avoiding mucous forming foods, which makes a lot of sense. If you've ever noticed how some food types can set you off, you're more aware than most people.
Now, this particular point may not address the black spots in mucous, but it's worth bearing in mind.