HELP!! Super gross & nasty! :/
Dreading Methods
I am not sure if it will help for you or not. I was only using a baking soda rinse for my locks for a while and recently SE suggested I use sea salt also. It worked wonders. I had the same issue with my locks. I was also having huge issues with scalp build up. After two rinses of baking soda and sea salt my problems were solved. The first time I could not believe how much dirt came from my locks. I think the dirt that muddled the water when I washed my lockswas actually that NASTY grime you speak of, only the rinse dissolved it from mty dreadies. Best of luck on your journey! Hope this helps 
Take care Spirit!