The bones retrieved from the La Brea tar pits are stained an impressive greasy black. That is from sitting in the tar after they died. If your creatures had circulating tar-like substances in their blood from their food supply, their bones might be stained black in life.
Asphaltenes is the catchall for these persistent gooey hydrocarbons as they occur in tar pits and elsewhere.
Asphaltenes consist primarily of carbon, hydrogen, nitrogen, oxygen,
and sulfur, as well as trace amounts of vanadium and nickel...
Asphaltenes are defined operationally as the n-heptane insoluble,
toluene soluble component of a carbonaceous material such as crude
oil, bitumen, or coal. Asphaltenes have been shown to have a
distribution of molecular masses in the range of 400 u to 1500 u, but
the average and maximum values are difficult to determine due to
aggregation of the molecules in solution. The molecular structure of
asphaltenes is difficult to determine because the molecules tend to
stick together in solution. These materials are extremely complex
mixtures containing hundreds or even thousands of individual chemical
I could imagine circulating globs of asphaltene might contribute to accelerated blood clotting, or serve an immunogenic role in sticking to and inactivating parasites. Imagine a mosquito drying to drink blood containing this tenacious goo.
In addition to black bones, your creatures would smell like tar and have thick black blood. If you heated it enough, once you boiled off the water the residual asphaltenes would burn and probably produce a lot of dirty brown smoke.