See if a local lab can analyze the stuff. This will give clues to where it could be coming from.
Another great resource is contact firms like "Disaster Restoration Services (DRS)" or "Restorex". Look in the yellow pages. I don't know who is in your area. Your homeowners agent might recommend somone, however, you do not have to go with who they recommend.
Last year, I had a problem so I contacted my homeowners agent and they recommend someone. I didn't agree with there solution, so I called DRS. DRS determined that the tar looking substance is "Artillary Fungus" growing in my mulch. It shoots tar looking blobs for up to 20 feet. And the fungus is photosensitive so it aims for my white porch ceiling. Without these guys, I would be guessing for the next 20 years! I then did some web searching on Artilliary Fungus and it is EXACTLY what I have been experiencing.
These guys see everything. So they may have seen what you are experiencing. The rest of us are just a bunch of dummys & we would only be guessing.