Pretty With Pink

For years I used a tennis ball to reach that certain spot in my back that's always causing me pain. At the gym we have a blackroll circle with several stations of blackrolls and I always use it (and it helps and feels good).
Now they sold blackrolls, blackrollballs and blackrolltwinballs at Aldi and I invested a few €uros. The twins came with a single brother sister.

Blackrolltwinballs - I like that word - it rolls in my throat like the ball on my back.

But why is it called a blackroll when it's pink and a ball?

19:35; 7°C
rain's warmer now