This sounds like what I'm getting in the pre-filter in my '80 300TD; since May 1, 2004 I've been burning blends of WVO settled and filtered to 0.5 micron. Lately I've been running 90% WVO and 10% RUG. It's going fine, but I'm still getting the black stuff in the pre-filter. When this first started happening I thought it might be restricted flow due to the viscosity and responded by adding diesel #2 or RUG. The problem did not seem to be related to cool weather (Central California summer). Reducing the viscosity sometimes seemed to help but didn't really solve it. What has solved it every time is changing the filter. They're cheap and take 5 minutes to swap. I fill the new one with WVO and I'm back on the road.
It's pretty well-known that biodiesel causes this by dissolving or loosening deposits from inside tank surfaces. I've heard it said that WVO can do this too. I'm hoping it will diminish with time. I'm hoping it isn't dissolving any rubber hoses or other rubber parts. I want to remove and inspect some of these, such as the injector leak-off hoses and tank-to-fuel- line hoses. I recently added some Biobor in case "algae" is involved. I've had some success with cleaning out the used filters; I carry spares, and just ordered 10 from Fastlane. I checked the tank screen some time ago and found it clean; check archives ("tank screen") for my technique with tank full. I did replace the main filter early on as I didn't know its age. I bought the car last October, and the odometer was stuck until I fixed it a couple of months ago, thanks to advice on this forum. Search more, and listen to P.E. Haiges for the easier way to find out if the tank screen is blocked.