It might have been better to have saved a sample of the stuff and have it closely examined—chemically or microscopically. Then, you might have deduced what it is; and next, whence it came, etc. Nevertheless, black residues suggest carbon content from burning or other type of degradation, which could come from nearly anything these days. Another idea is to contact the previous owner, through the broker if necessary, to inquire if it has been seen before now.
Also, re:mixing elbow. My Yanmar came with a SS elbow still fine after 20 yr. So, there is probably a Yanmar part in stainless.