Some things that thrive in rot and decay can become saturated in black anima, and it can be harvested from such things... the most common source of black anima to be so harvested are fungi that grow in dark and rotten bogs or upon corpses, but it can also accumulates in things that feed for prolonged periods on carrion, rot, and decay. Aquatic bottom feeders, rodents, and many vermin accumulate black anima over time, although harvesting black anima from things is normally impractical. These absorbers of black anima often are transformed over time into dark things, and this is the true source for many strange abominations that lurk in the remote and dark bogs, crypts, and caverns. Malevolent oozes, slimes, and fungi; aquatic monstrosities, strange vermin and carrion beasts; all often blamed on mad wizards, but most all have been borne out of humble creatures corrupted by black anima.
I see correlations between black anima as a magical concept, and petroleum... both are created through a corrupted or incomplete decomposition process that forces the stored energies of the source material to be transformed instead, as a toxic corruption of what it began as.