Wow, Jim, thanks for all the effort. I think I'll just give up eating fish all together. Right!
Actually, dos ballenas has offered to analyze some tissue samples. I haven't smelled the stuff but I will and I'll also see if it burns. I don't think it's tar but your theory on an exterior parasite creating pockets of concentrated waste inside of a fish seems reasonable. I just didn't see anything similar to that monster hanging out of your YT, just the usual nematode type worms inside. Blood clots caused by internal parasites seems more likely, or as I think you suggest, encysted clumps of parasites.
The black stuff seems to be concentrated around the gut. I understand that ingested parasites often migrate from the gut to the flesh surrounding it. As I have cut the big chunks into smaller ones, it seems that there is plenty of clean meat further away from the gut.
I guess I'd better do some dissecting. I'd like to hear what our resident researcher finds out. I'll let you know what happens.
As for the other photos, that's just showing off.
Steve