"polyethylene resin
These blackwater tanks are made from rotationally-molded polyethylene resin that provides strength, long service life, and great protection against cracks, rust, and corrosion. Blackwater holding tanks are manufactured by Dura-Cast and Ronco. RV septic tanks can be used for most recreational vehicles makes and models."
ABS tanks have kind of gone away years ago. Most tanks are this material above and COLOR has no bearing on what material they are made of. A good tell if you suspect ABS is to look at the 3 inch exit fitting of the tank. ABS tanks will show/have ABS cement where the pipe goes in. IF you have a Hose Clamp, it is not ABS. Doug